diff --git a/application/controllers/Booking.php b/application/controllers/Booking.php
index ceeb6418..72398059 100755
--- a/application/controllers/Booking.php
+++ b/application/controllers/Booking.php
@@ -131,7 +131,6 @@ class Booking extends EA_Controller {
         $display_login_button = setting('display_login_button');
         $display_delete_personal_information = setting('display_delete_personal_information');
         $book_advance_timeout = setting('book_advance_timeout');
-        $future_booking_limit = setting('future_booking_limit');
         $theme = request('theme', setting('theme', 'default'));
 
         if (empty($theme) || ! file_exists(__DIR__ . '/../../assets/css/themes/' . $theme . '.min.css'))
@@ -286,7 +285,7 @@ class Booking extends EA_Controller {
      *
      * @param string $appointment_hash
      */
-    public function reschedule($appointment_hash)
+    public function reschedule(string $appointment_hash)
     {
         html_vars(['appointment_hash' => $appointment_hash]);
 
@@ -311,7 +310,7 @@ class Booking extends EA_Controller {
 
             if (empty($provider_id))
             {
-                json_response([]);
+                json_response();
 
                 return;
             }
@@ -397,7 +396,7 @@ class Booking extends EA_Controller {
                     // Check if the provider is available for the requested date.
                     $available_hours = $this->availability->get_available_hours($date, $service, $provider);
 
-                    if (count($available_hours) > $max_hours_count && (empty($hour) || in_array($hour, $available_hours, FALSE)))
+                    if (count($available_hours) > $max_hours_count && (empty($hour) || in_array($hour, $available_hours)))
                     {
                         $provider_id = $provider['id'];
 
@@ -567,7 +566,7 @@ class Booking extends EA_Controller {
                 'time_format' => setting('time_format')
             ];
 
-            $this->synchronization->sync_appointment_saved($appointment, $service, $provider, $customer, $settings, $manage_mode);
+            $this->synchronization->sync_appointment_saved($appointment, $service, $provider, $customer, $settings);
 
             $this->notifications->notify_appointment_saved($appointment, $service, $provider, $customer, $settings, $manage_mode);
 
@@ -596,7 +595,7 @@ class Booking extends EA_Controller {
      * Use this method just before the customer confirms the appointment registration. If the selected date was reserved
      * in the meanwhile, the customer must be prompted to select another time.
      *
-     * @return int Returns the ID of the provider that is available for the appointment.
+     * @return int|null Returns the ID of the provider that is available for the appointment.
      *
      * @throws Exception
      */
diff --git a/application/controllers/Booking_confirmation.php b/application/controllers/Booking_confirmation.php
index 4ca87e7e..64300f11 100755
--- a/application/controllers/Booking_confirmation.php
+++ b/application/controllers/Booking_confirmation.php
@@ -36,6 +36,8 @@ class Booking_confirmation extends EA_Controller {
 
     /**
      * Display the appointment registration success page.
+     * 
+     * @throws Exception
      */
     public function of()
     {
diff --git a/application/controllers/Calendar.php b/application/controllers/Calendar.php
index b1bdd84f..cee3c718 100644
--- a/application/controllers/Calendar.php
+++ b/application/controllers/Calendar.php
@@ -227,7 +227,7 @@ class Calendar extends EA_Controller {
                     ? can('add', PRIV_APPOINTMENTS)
                     : can('edit', PRIV_APPOINTMENTS);
 
-                if ($required_permissions == FALSE)
+                if ( ! $required_permissions)
                 {
                     throw new RuntimeException('You do not have the required permissions for this task.');
                 }
@@ -279,7 +279,7 @@ class Calendar extends EA_Controller {
                 'time_format' => setting('time_format')
             ];
 
-            $this->synchronization->sync_appointment_saved($appointment, $service, $provider, $customer, $settings, $manage_mode);
+            $this->synchronization->sync_appointment_saved($appointment, $service, $provider, $customer, $settings);
 
             $this->notifications->notify_appointment_saved($appointment, $service, $provider, $customer, $settings, $manage_mode);
 
@@ -411,7 +411,7 @@ class Calendar extends EA_Controller {
 
             $this->unavailabilities_model->delete($unavailability_id);
 
-            $this->synchronization->sync_appointment_deleted($unavailability, $provider);
+            $this->synchronization->sync_unavailability_deleted($unavailability, $provider);
 
             $this->webhooks_client->trigger(WEBHOOK_UNAVAILABILITY_DELETE, $unavailability);
 
diff --git a/application/controllers/Google.php b/application/controllers/Google.php
index c75af08f..7ec2c305 100644
--- a/application/controllers/Google.php
+++ b/application/controllers/Google.php
@@ -39,8 +39,6 @@ class Google extends EA_Controller {
      * This method will completely sync the appointments of a provider with his Google Calendar account. The sync period
      * needs to be relatively small, because a lot of API calls might be necessary and this will lead to consuming the
      * Google limit for the Calendar API usage.
-     *
-     * @param string $provider_id Provider record to be synced.
      */
     public static function sync(string $provider_id = NULL)
     {
@@ -166,7 +164,7 @@ class Google extends EA_Controller {
                         }
 
                     }
-                    catch (Throwable $e)
+                    catch (Throwable)
                     {
                         // Appointment not found on Google Calendar, delete from Easy!Appointments.
                         $CI->appointments_model->delete($appointment['id']);
@@ -285,7 +283,10 @@ class Google extends EA_Controller {
      *
      * IMPORTANT: Because it is necessary to authorize the application using the web server flow (see official
      * documentation of OAuth), every Easy!Appointments installation should use its own calendar api key. So in every
-     * api console account, the "http://path-to-Easy!Appointments/google/oauth_callback" should be included in an allowed redirect URL.
+     * api console account, the "http://path-to-Easy!Appointments/google/oauth_callback" should be included in an 
+     * allowed redirect URL.
+     * 
+     * @throws Exception
      */
     public function oauth_callback()
     {
@@ -431,7 +432,7 @@ class Google extends EA_Controller {
 
             $this->providers_model->set_setting($provider_id, 'google_sync', FALSE);
 
-            $this->providers_model->set_setting($provider_id, 'google_token', NULL);
+            $this->providers_model->set_setting($provider_id, 'google_token');
 
             $this->appointments_model->clear_google_sync_ids($provider_id);
 
diff --git a/application/controllers/Installation.php b/application/controllers/Installation.php
index 2231f409..f274e3b0 100644
--- a/application/controllers/Installation.php
+++ b/application/controllers/Installation.php
@@ -42,7 +42,7 @@ class Installation extends EA_Controller {
     {
         if (is_app_installed())
         {
-            redirect('');
+            redirect();
             return;
         }
 
diff --git a/application/controllers/api/v1/Appointments_api_v1.php b/application/controllers/api/v1/Appointments_api_v1.php
index 056b7143..1490fbe7 100644
--- a/application/controllers/api/v1/Appointments_api_v1.php
+++ b/application/controllers/api/v1/Appointments_api_v1.php
@@ -334,7 +334,7 @@ class Appointments_api_v1 extends EA_Controller {
             'time_format' => setting('time_format')
         ];
 
-        $this->synchronization->sync_appointment_saved($appointment, $service, $provider, $customer, $settings, $manage_mode);
+        $this->synchronization->sync_appointment_saved($appointment, $service, $provider, $customer, $settings);
 
         $this->notifications->notify_appointment_saved($appointment, $service, $provider, $customer, $settings, $manage_mode);
     }
diff --git a/application/core/EA_Model.php b/application/core/EA_Model.php
index 0cebcff4..a877b5fc 100644
--- a/application/core/EA_Model.php
+++ b/application/core/EA_Model.php
@@ -43,7 +43,7 @@ class EA_Model extends CI_Model {
     /**
      * @var array
      */
-    protected $casts = [];
+    protected array $casts = [];
 
     /**
      * EA_Model constructor.
diff --git a/application/helpers/asset_helper.php b/application/helpers/asset_helper.php
index b75ea094..8599b56f 100644
--- a/application/helpers/asset_helper.php
+++ b/application/helpers/asset_helper.php
@@ -22,18 +22,18 @@
  *
  * @return string Returns the final asset URL.
  */
-function asset_url($uri = '', $protocol = NULL)
+function asset_url(string $uri = '', string $protocol = NULL): string
 {
     $debug = config('debug');
 
     $cache_busting_token = ! $debug ? '?' . config('cache_busting_token') : '';
 
-    if (strpos(basename($uri), '.js') !== FALSE && strpos(basename($uri), '.min.js') === FALSE && ! $debug)
+    if (str_contains(basename($uri), '.js') && ! str_contains(basename($uri), '.min.js') && ! $debug)
     {
         $uri = str_replace('.js', '.min.js', $uri);
     }
 
-    if (strpos(basename($uri), '.css') !== FALSE && strpos(basename($uri), '.min.css') === FALSE && ! $debug)
+    if (str_contains(basename($uri), '.css') && ! str_contains(basename($uri), '.min.css') && ! $debug)
     {
         $uri = str_replace('.css', '.min.css', $uri);
     }
diff --git a/application/helpers/config_helper.php b/application/helpers/config_helper.php
index 2f86c6f5..81414fbc 100644
--- a/application/helpers/config_helper.php
+++ b/application/helpers/config_helper.php
@@ -25,13 +25,13 @@
  * config(['version' => '1.0.0']);
  *
  * @param array|string $key Configuration key.
- * @param mixed $default Default value in case the requested config has no value.
+ * @param mixed|null $default Default value in case the requested config has no value.
  *
  * @return mixed|NULL Returns the requested value or NULL if you assign a new configuration value.
  *
  * @throws InvalidArgumentException
  */
-function config($key, $default = NULL)
+function config(array|string $key, mixed $default = NULL): mixed
 {
     /** @var EA_Controller $CI */
     $CI = &get_instance();
@@ -71,14 +71,14 @@ if ( ! function_exists('script_vars'))
      *
      * script_vars(['version' => '1.0.0']);
      *
-     * @param array|string $key Configuration key.
-     * @param mixed $default Default value in case the requested config has no value.
+     * @param array|string|null $key Configuration key.
+     * @param mixed|null $default Default value in case the requested config has no value.
      *
      * @return mixed|NULL Returns the requested value or NULL if you assign a new configuration value.
      *
      * @throws InvalidArgumentException
      */
-    function script_vars($key = NULL, $default = NULL)
+    function script_vars(array|string $key = NULL, mixed $default = NULL): mixed
     {
         $script_vars = config('script_vars', []);
 
@@ -120,14 +120,14 @@ if ( ! function_exists('html_vars'))
      *
      * html_vars(['title' => 'Test Title']);
      *
-     * @param array|string $key Variable key.
-     * @param mixed $default Default value in case the requested variable has no value.
+     * @param array|string|null $key Variable key.
+     * @param mixed|null $default Default value in case the requested variable has no value.
      *
      * @return mixed|NULL Returns the requested value or NULL if you assign a new configuration value.
      *
      * @throws InvalidArgumentException
      */
-    function html_vars($key = NULL, $default = NULL)
+    function html_vars(array|string $key = NULL, mixed $default = NULL): mixed
     {
         $html_vars = config('html_vars', []);
 
@@ -169,14 +169,14 @@ if ( ! function_exists('vars'))
      *
      * vars(['version' => '1.0.0']);
      *
-     * @param array|string $key Configuration key.
-     * @param mixed $default Default value in case the requested config has no value.
+     * @param array|string|null $key Configuration key.
+     * @param mixed|null $default Default value in case the requested config has no value.
      *
      * @return mixed|NULL Returns the requested value or NULL if you assign a new configuration value.
      *
      * @throws InvalidArgumentException
      */
-    function vars($key = NULL, $default = NULL)
+    function vars(array|string $key = NULL, mixed $default = NULL): mixed
     {
         return html_vars($key) ?? script_vars($key) ?? $default;
     }
diff --git a/application/helpers/debug_helper.php b/application/helpers/debug_helper.php
index 62fdf1f5..4f1d248b 100644
--- a/application/helpers/debug_helper.php
+++ b/application/helpers/debug_helper.php
@@ -1,4 +1,6 @@
-<?php defined('BASEPATH') or exit('No direct script access allowed');
+<?php use JetBrains\PhpStorm\NoReturn;
+
+defined('BASEPATH') or exit('No direct script access allowed');
 
 /* ----------------------------------------------------------------------------
  * Easy!Appointments - Online Appointment Scheduler
@@ -22,7 +24,7 @@ if ( ! function_exists('dd'))
      *
      * @param mixed ...$vars
      */
-    function dd(...$vars)
+    #[NoReturn] function dd(...$vars): void
     {
         var_dump($vars);
 
diff --git a/application/helpers/env_helper.php b/application/helpers/env_helper.php
index fb6fe0ff..d5984c0a 100644
--- a/application/helpers/env_helper.php
+++ b/application/helpers/env_helper.php
@@ -21,13 +21,13 @@ if ( ! function_exists('env'))
      * $debug = env('debug', FALSE);
      *
      * @param string $key Environment key. 
-     * @param mixed $default Default value in case the requested variable has no value.
+     * @param mixed|null $default Default value in case the requested variable has no value.
      *
      * @return mixed
      *
      * @throws InvalidArgumentException
      */
-    function env(string $key, $default = NULL)
+    function env(string $key, mixed $default = NULL): mixed
     {
         if (empty($key))
         {
diff --git a/application/helpers/html_helper.php b/application/helpers/html_helper.php
index 6ac0fb8b..377bf9ab 100644
--- a/application/helpers/html_helper.php
+++ b/application/helpers/html_helper.php
@@ -48,9 +48,9 @@ if ( ! function_exists('component'))
      * @param array $vars Additional parameters for the component.
      * @param bool $return Whether to return the HTML or echo it directly.
      *
-     * @return string Return the HTML if the $return argument is TRUE or NULL.
+     * @return mixed Return the HTML if the $return argument is TRUE or NULL.
      */
-    function component(string $component, array $vars = [], bool $return = FALSE)
+    function component(string $component, array $vars = [], bool $return = FALSE): mixed
     {
         /** @var EA_Controller $CI */
         $CI = get_instance();
@@ -66,7 +66,7 @@ if ( ! function_exists('extend'))
      *
      * @param $layout
      */
-    function extend($layout)
+    function extend($layout): void
     {
         config([
             'layout' => [
@@ -99,7 +99,7 @@ if ( ! function_exists('section'))
      *
      * @param string $name
      */
-    function section(string $name)
+    function section(string $name): void
     {
         $layout = config('layout');
 
@@ -148,7 +148,7 @@ if ( ! function_exists('end_section'))
      *
      * @param string $name
      */
-    function end_section(string $name)
+    function end_section(string $name): void
     {
         $layout = config('layout');
 
@@ -170,7 +170,7 @@ if ( ! function_exists('slot'))
      *
      * @param string $name
      */
-    function slot(string $name)
+    function slot(string $name): void
     {
         $layout = config('layout');
 
diff --git a/application/helpers/installation_helper.php b/application/helpers/installation_helper.php
index c65d56c9..535593a8 100644
--- a/application/helpers/installation_helper.php
+++ b/application/helpers/installation_helper.php
@@ -21,7 +21,7 @@
  *
  * @return bool Returns whether E!A is installed or not.
  */
-function is_app_installed()
+function is_app_installed(): bool
 {
     $CI =& get_instance();
 
diff --git a/application/helpers/rate_limit_helper.php b/application/helpers/rate_limit_helper.php
index 3d932aab..271b58ae 100644
--- a/application/helpers/rate_limit_helper.php
+++ b/application/helpers/rate_limit_helper.php
@@ -26,8 +26,9 @@ if ( ! function_exists('rate_limit'))
      * @param int $max_requests Number of allowed requests, defaults to 100.
      * @param int $duration In seconds, defaults to 2 minutes.
      */
-    function rate_limit(string $ip, int $max_requests = 100, int $duration = 120)
+    function rate_limit(string $ip, int $max_requests = 100, int $duration = 120): void
     {
+        /** @var EA_Controller $CI */
         $CI =& get_instance();
 
         $rate_limiting = $CI->config->item('rate_limiting');
diff --git a/application/helpers/routes_helper.php b/application/helpers/routes_helper.php
index d131bbfe..3554e9f5 100644
--- a/application/helpers/routes_helper.php
+++ b/application/helpers/routes_helper.php
@@ -18,9 +18,9 @@ if ( ! function_exists('route_api_resource'))
      *
      * @param array $route Route config.
      * @param string $resource Resource name.
-     * @param string|null $prefix URL prefix (e.g. api/v1/).
+     * @param string $prefix URL prefix (e.g. api/v1/).
      */
-    function route_api_resource(array &$route, string $resource, string $prefix = '')
+    function route_api_resource(array &$route, string $resource, string $prefix = ''): void
     {
         $route[$prefix . $resource]['post'] = 'api/v1/' . $resource . '_api_v1/store';
         $route[$prefix . $resource . '/(:num)']['put'] = 'api/v1/' . $resource . '_api_v1/update/$1';
diff --git a/application/helpers/session_helper.php b/application/helpers/session_helper.php
index f22e67a4..0af7cf79 100644
--- a/application/helpers/session_helper.php
+++ b/application/helpers/session_helper.php
@@ -26,14 +26,14 @@ if ( ! function_exists('session'))
      *
      * session(['logged_in' => FALSE]);
      *
-     * @param array|string $key Session item key. 
-     * @param mixed $default Default value in case the requested session item has no value.
+     * @param array|string|null $key Session item key. 
+     * @param mixed|null $default Default value in case the requested session item has no value.
      *
      * @return mixed|NULL Returns the requested value or NULL if you assign a new session value.
      *
      * @throws InvalidArgumentException
      */
-    function session($key = NULL, $default = NULL)
+    function session(array|string $key = NULL, mixed $default = NULL): mixed
     {
         /** @var EA_Controller $CI */
         $CI = &get_instance();
diff --git a/application/helpers/setting_helper.php b/application/helpers/setting_helper.php
index c1b428d1..e1332bdd 100644
--- a/application/helpers/setting_helper.php
+++ b/application/helpers/setting_helper.php
@@ -26,14 +26,14 @@ if ( ! function_exists('setting'))
      *
      * setting(['company_name' => 'ACME Inc']);
      *
-     * @param array|string $key Setting key.
-     * @param mixed $default Default value in case the requested setting has no value.
+     * @param array|string|null $key Setting key.
+     * @param mixed|null $default Default value in case the requested setting has no value.
      *
      * @return mixed|NULL Returns the requested value or NULL if you assign a new setting value.
      *
      * @throws InvalidArgumentException
      */
-    function setting($key = NULL, $default = NULL)
+    function setting(array|string $key = NULL, mixed $default = NULL): mixed
     {
         /** @var EA_Controller $CI */
         $CI = &get_instance();
diff --git a/application/libraries/Accounts.php b/application/libraries/Accounts.php
index f2c3e679..706604d7 100644
--- a/application/libraries/Accounts.php
+++ b/application/libraries/Accounts.php
@@ -21,9 +21,9 @@
  */
 class Accounts {
     /**
-     * @var EA_Controller
+     * @var EA_Controller|CI_Controller
      */
-    protected $CI;
+    protected EA_Controller|CI_Controller $CI;
 
     /**
      * Accounts constructor.
@@ -45,6 +45,7 @@ class Accounts {
      * @param string $password Password (non-hashed).
      *
      * @return array|null Returns an associative array with the PHP session data or NULL on failure.
+     * @throws Exception
      */
     public function check_login(string $username, string $password): ?array
     {
@@ -112,9 +113,9 @@ class Accounts {
      * @param string $username Username.
      * @param string $email Email.
      *
-     * @return string|bool Returns the new password on success or FALSE on failure.
+     * @return string Returns the new password on success or FALSE on failure.
      *
-     * @throws RuntimeException
+     * @throws Exception
      */
     public function regenerate_password(string $username, string $email): string
     {
diff --git a/application/libraries/Api.php b/application/libraries/Api.php
index cc9484f4..eaf2fff6 100644
--- a/application/libraries/Api.php
+++ b/application/libraries/Api.php
@@ -1,4 +1,6 @@
-<?php defined('BASEPATH') or exit('No direct script access allowed');
+<?php use JetBrains\PhpStorm\NoReturn;
+
+defined('BASEPATH') or exit('No direct script access allowed');
 
 /* ----------------------------------------------------------------------------
  * Easy!Appointments - Online Appointment Scheduler
@@ -21,19 +23,19 @@
  */
 class Api {
     /**
-     * @var EA_Controller
+     * @var EA_Controller|CI_Controller
      */
-    protected $CI;
+    protected EA_Controller|CI_Controller $CI;
 
     /**
      * @var int
      */
-    protected $default_length = 20;
+    protected int $default_length = 20;
 
     /**
      * @var EA_Model
      */
-    protected $model;
+    protected EA_Model $model;
 
     /**
      * Api constructor.
@@ -50,7 +52,7 @@ class Api {
      *
      * @param string $model
      */
-    public function model(string $model)
+    public function model(string $model): void
     {
         $this->CI->load->model($model);
 
@@ -60,7 +62,7 @@ class Api {
     /**
      * Authorize the API request (Basic Auth or Bearer Token supported).
      */
-    public function auth()
+    public function auth(): void
     {
         try
         {
@@ -84,7 +86,7 @@ class Api {
                 throw new RuntimeException('The provided credentials do not match any admin user!', 401, 'Unauthorized');
             }
         }
-        catch (Throwable $e)
+        catch (Throwable)
         {
             $this->request_authentication();
         }
@@ -93,7 +95,7 @@ class Api {
     /**
      * Returns the bearer token value.
      *
-     * @return string
+     * @return string|null
      */
     protected function get_bearer_token(): ?string
     {
@@ -153,7 +155,7 @@ class Api {
     /**
      * Sets request authentication headers.
      */
-    public function request_authentication()
+    #[NoReturn] public function request_authentication(): void
     {
         header('WWW-Authenticate: Basic realm="Easy!Appointments"');
         header('HTTP/1.0 401 Unauthorized');
diff --git a/application/libraries/Availability.php b/application/libraries/Availability.php
index 2fd9b557..0372e380 100644
--- a/application/libraries/Availability.php
+++ b/application/libraries/Availability.php
@@ -21,9 +21,9 @@
  */
 class Availability {
     /**
-     * @var EA_Controller
+     * @var EA_Controller|CI_Controller
      */
-    protected $CI;
+    protected EA_Controller|CI_Controller $CI;
 
     /**
      * Availability constructor.
@@ -126,7 +126,7 @@ class Availability {
 
         $date_working_plan = $working_plan[$working_day] ?? NULL;
 
-        // Search if the $date is an custom availability period added outside the normal working plan.
+        // Search if the $date is a custom availability period added outside the normal working plan.
         if (isset($working_plan_exceptions[$date]))
         {
             $date_working_plan = $working_plan_exceptions[$date];
@@ -145,7 +145,7 @@ class Availability {
             $day_end = new DateTime($date_working_plan['end']);
 
             // Split the working plan to available time periods that do not contain the breaks in them.
-            foreach ($date_working_plan['breaks'] as $index => $break)
+            foreach ($date_working_plan['breaks'] as $break)
             {
                 $break_start = new DateTime($break['start']);
                 $break_end = new DateTime($break['end']);
@@ -373,7 +373,7 @@ class Availability {
 
         $date_working_plan = $working_plan[$working_day] ?? NULL;
 
-        // Search if the $date is an custom availability period added outside the normal working plan.
+        // Search if the $date is a custom availability period added outside the normal working plan.
         if (isset($working_plan_exceptions[$date]))
         {
             $date_working_plan = $working_plan_exceptions[$date];
@@ -505,7 +505,6 @@ class Availability {
                 {
                     // break contains period
                     $period['start'] = $break_end;
-                    continue;
                 }
             }
         }
@@ -620,7 +619,7 @@ class Availability {
      * @param array $available_hours
      * @param array $provider
      *
-     * @return array|mixed
+     * @return array
      *
      * @throws Exception
      */
diff --git a/application/libraries/Email_messages.php b/application/libraries/Email_messages.php
index af4d5376..a8e4dbee 100644
--- a/application/libraries/Email_messages.php
+++ b/application/libraries/Email_messages.php
@@ -20,9 +20,9 @@
  */
 class Email_messages {
     /**
-     * @var EA_Controller
+     * @var EA_Controller|CI_Controller
      */
-    protected $CI;
+    protected EA_Controller|CI_Controller $CI;
 
     /**
      * Email_messages constructor.
diff --git a/application/libraries/Google_sync.php b/application/libraries/Google_sync.php
index d0046773..0b66bc9c 100644
--- a/application/libraries/Google_sync.php
+++ b/application/libraries/Google_sync.php
@@ -22,19 +22,19 @@ use Google\Service\Calendar\Events;
  */
 class Google_sync {
     /**
-     * @var EA_Controller
+     * @var EA_Controller|CI_Controller
      */
-    protected $CI;
+    protected EA_Controller|CI_Controller $CI;
 
     /**
      * @var Google_Client
      */
-    protected $client;
+    protected Google_Client $client;
 
     /**
      * @var Google_Service_Calendar
      */
-    protected $service;
+    protected Google_Service_Calendar $service;
 
     /**
      * Google_sync constructor.
@@ -57,7 +57,7 @@ class Google_sync {
     /**
      * Initialize the client, so that existing execution errors are not passed from one provider to another.
      */
-    public function initialize_clients()
+    public function initialize_clients(): void
     {
         $http = new GuzzleHttp\Client([
             'verify' => FALSE
@@ -126,7 +126,7 @@ class Google_sync {
      * @param string $refresh_token The provider's refresh token. This value is stored in the database and used every
      * time we need to make actions to his Google Calendar account.
      */
-    public function refresh_token(string $refresh_token)
+    public function refresh_token(string $refresh_token): void
     {
         $this->initialize_clients();
 
@@ -247,7 +247,7 @@ class Google_sync {
      * @param array $provider Provider data.
      * @param string $google_event_id The Google Calendar event ID to be removed.
      */
-    public function delete_appointment(array $provider, string $google_event_id)
+    public function delete_appointment(array $provider, string $google_event_id): void
     {
         $this->service->events->delete($provider['settings']['google_calendar'], $google_event_id);
     }
@@ -318,7 +318,7 @@ class Google_sync {
      * @param array $provider Provider data.
      * @param string $google_event_id Google Calendar event ID to be removed.
      */
-    public function delete_unavailability(array $provider, string $google_event_id)
+    public function delete_unavailability(array $provider, string $google_event_id): void
     {
         $this->service->events->delete($provider['settings']['google_calendar'], $google_event_id);
     }
diff --git a/application/libraries/Ics_file.php b/application/libraries/Ics_file.php
index 25789d22..fb900446 100644
--- a/application/libraries/Ics_file.php
+++ b/application/libraries/Ics_file.php
@@ -33,9 +33,9 @@ use Jsvrcek\ICS\Utility\Formatter;
  */
 class Ics_file {
     /**
-     * @var EA_Controller
+     * @var EA_Controller|CI_Controller
      */
-    protected $CI;
+    protected EA_Controller|CI_Controller $CI;
 
     /**
      * Availability constructor.
diff --git a/application/libraries/Instance.php b/application/libraries/Instance.php
index 05d24767..c19db101 100644
--- a/application/libraries/Instance.php
+++ b/application/libraries/Instance.php
@@ -22,9 +22,9 @@ require_once __DIR__ . '/../core/EA_Migration.php';
  */
 class Instance {
     /**
-     * @var EA_Controller
+     * @var EA_Controller|CI_Controller
      */
-    protected $CI;
+    protected EA_Controller|CI_Controller $CI;
 
     /**
      * Installation constructor.
@@ -47,7 +47,7 @@ class Instance {
      *
      * @param string $type Provide "fresh" to revert previous migrations and start from the beginning or "up"/"down" to step.
      */
-    public function migrate(string $type = '')
+    public function migrate(string $type = ''): void
     {
         $current_version = $this->CI->migration->current_version();
 
@@ -166,7 +166,7 @@ class Instance {
      *
      * @throws Exception
      */
-    public function backup(string $path = NULL)
+    public function backup(string $path = NULL): void
     {
         $path = $path ?? APPPATH . '/../storage/backups';
 
diff --git a/application/libraries/Notifications.php b/application/libraries/Notifications.php
index 3ad55974..7fd294a8 100644
--- a/application/libraries/Notifications.php
+++ b/application/libraries/Notifications.php
@@ -20,9 +20,9 @@
  */
 class Notifications {
     /**
-     * @var EA_Controller
+     * @var EA_Controller|CI_Controller
      */
-    protected $CI;
+    protected EA_Controller|CI_Controller $CI;
 
     /**
      * Notifications constructor.
diff --git a/application/libraries/Permissions.php b/application/libraries/Permissions.php
index ddf8368d..2dd12bab 100644
--- a/application/libraries/Permissions.php
+++ b/application/libraries/Permissions.php
@@ -21,9 +21,9 @@
  */
 class Permissions {
     /**
-     * @var EA_Controller
+     * @var EA_Controller|CI_Controller
      */
-    protected $CI;
+    protected EA_Controller|CI_Controller $CI;
 
     /**
      * Permissions constructor.
@@ -65,12 +65,12 @@ class Permissions {
             return TRUE;
         }
 
-        if ($role_slug === DB_SLUG_PROVIDER && $limit_customer_access)
+        if ($role_slug === DB_SLUG_PROVIDER)
         {
             return $this->CI->appointments_model->query()->where(['id_users_provider' => $user_id, 'id_users_customer' => $customer_id])->get()->num_rows() > 0;
         }
 
-        if ($role_slug === DB_SLUG_SECRETARY && $limit_customer_access)
+        if ($role_slug === DB_SLUG_SECRETARY)
         {
             $secretary = $this->CI->secretaries_model->find($user_id);
 
diff --git a/application/libraries/Synchronization.php b/application/libraries/Synchronization.php
index a99ac860..b1f460a0 100644
--- a/application/libraries/Synchronization.php
+++ b/application/libraries/Synchronization.php
@@ -20,9 +20,9 @@
  */
 class Synchronization {
     /**
-     * @var EA_Controller
+     * @var EA_Controller|CI_Controller
      */
-    protected $CI;
+    protected EA_Controller|CI_Controller $CI;
 
     /**
      * Synchronization constructor.
@@ -45,9 +45,8 @@ class Synchronization {
      * @param array $provider Provider record.
      * @param array $customer Customer record.
      * @param array $settings Required settings for the notification content.
-     * @param bool|false $manage_mode True if the appointment is being edited.
      */
-    public function sync_appointment_saved(array $appointment, array $service, array $provider, array $customer, array $settings, bool $manage_mode = FALSE)
+    public function sync_appointment_saved(array $appointment, array $service, array $provider, array $customer, array $settings): void
     {
         try
         {
@@ -103,7 +102,7 @@ class Synchronization {
      * @param array $appointment Appointment record.
      * @param array $provider Provider record.
      */
-    public function sync_appointment_deleted(array $appointment, array $provider)
+    public function sync_appointment_deleted(array $appointment, array $provider): void
     {
         try
         {
@@ -137,7 +136,7 @@ class Synchronization {
      * @param array $unavailability Unavailability record.
      * @param array $provider Provider record.
      */
-    public function sync_unavailability_saved(array $unavailability, array $provider)
+    public function sync_unavailability_saved(array $unavailability, array $provider): void
     {
         try
         {
@@ -181,7 +180,7 @@ class Synchronization {
      * @param array $unavailability Unavailability record.
      * @param array $provider Provider record.
      */
-    public function sync_unavailability_deleted(array $unavailability, array $provider)
+    public function sync_unavailability_deleted(array $unavailability, array $provider): void
     {
         try
         {
@@ -213,9 +212,9 @@ class Synchronization {
      * 
      * @throws Exception
      */
-    public function remove_appointment_on_provider_change($appointment_id)
+    public function remove_appointment_on_provider_change($appointment_id): void
     {
-        $existing_appointment = $this->CI->appointments_model->get_row($appointment_id);
+        $existing_appointment = $this->CI->appointments_model->find($appointment_id);
 
         $existing_google_id = $existing_appointment['id_google_calendar'];
 
@@ -223,7 +222,7 @@ class Synchronization {
 
         if ( ! empty($existing_google_id) && (int)$existing_provider_id !== (int)$existing_appointment['id_users_provider'])
         {
-            $existing_provider = $this->CI->providers_model->get_row($existing_provider_id);
+            $existing_provider = $this->CI->providers_model->find($existing_provider_id);
 
             if ($existing_provider['settings']['google_sync'])
             {
diff --git a/application/libraries/Timezones.php b/application/libraries/Timezones.php
index 58f9ef03..d041a53c 100644
--- a/application/libraries/Timezones.php
+++ b/application/libraries/Timezones.php
@@ -20,19 +20,19 @@
  */
 class Timezones {
     /**
-     * @var EA_Controller
+     * @var EA_Controller|CI_Controller
      */
-    protected $CI;
+    protected EA_Controller|CI_Controller $CI;
 
     /**
      * @var string
      */
-    protected $default = 'UTC';
+    protected string $default = 'UTC';
 
     /**
      * @var array
      */
-    protected $timezones = [
+    protected array $timezones = [
         'UTC' => [
             'UTC' => 'UTC'
         ],
@@ -517,31 +517,17 @@ class Timezones {
      *
      * @return array
      */
-    public function to_grouped_array()
+    public function to_grouped_array(): array
     {
         return $this->timezones;
     }
 
-    /**
-     * Returns the session timezone or the default timezone as a fallback.
-     *
-     * @return string
-     */
-    public function get_session_timezone(): string
-    {
-        $default_timezone = $this->get_default_timezone();
-
-        $timezone = session('timezone');
-
-        return $timezone ?? $default_timezone;
-    }
-
     /**
      * Get the default timezone value of the current system.
      *
      * @return string
      */
-    public function get_default_timezone()
+    public function get_default_timezone(): string
     {
         return 'UTC';
     }
@@ -557,7 +543,7 @@ class Timezones {
      *
      * @throws Exception
      */
-    public function convert($value, $from_timezone, $to_timezone)
+    public function convert(string $value, string $from_timezone, string $to_timezone): string
     {
         if ( ! $to_timezone || $from_timezone === $to_timezone)
         {
@@ -582,11 +568,11 @@ class Timezones {
      *
      * @return string|null
      */
-    public function get_timezone_name($value)
+    public function get_timezone_name(string $value): ?string
     {
         $timezones = $this->to_array();
 
-        return isset($timezones[$value]) ? $timezones[$value] : NULL;
+        return $timezones[$value] ?? NULL;
     }
 
     /**
@@ -594,7 +580,7 @@ class Timezones {
      *
      * @return array
      */
-    public function to_array()
+    public function to_array(): array
     {
         return array_merge(...array_values($this->timezones));
     }
diff --git a/application/libraries/Webhooks_client.php b/application/libraries/Webhooks_client.php
index b6a0cc0d..8d4ad19c 100644
--- a/application/libraries/Webhooks_client.php
+++ b/application/libraries/Webhooks_client.php
@@ -22,9 +22,9 @@ use GuzzleHttp\Client;
  */
 class Webhooks_client {
     /**
-     * @var EA_Controller
+     * @var EA_Controller|CI_Controller
      */
-    protected $CI;
+    protected EA_Controller|CI_Controller $CI;
 
     /**
      * Webhook client constructor.
@@ -56,7 +56,7 @@ class Webhooks_client {
 
         foreach ($webhooks as $webhook)
         {
-            if (strpos($webhook['actions'], $action) !== FALSE)
+            if (str_contains($webhook['actions'], $action))
             {
                 $this->call($webhook, $action, $payload);
             }
@@ -70,7 +70,7 @@ class Webhooks_client {
      * @param string $action
      * @param array $payload
      */
-    private function call(array $webhook, string $action, array $payload)
+    private function call(array $webhook, string $action, array $payload): void
     {
         try
         {
diff --git a/application/models/Admins_model.php b/application/models/Admins_model.php
index 8d2813fa..2086ec24 100644
--- a/application/models/Admins_model.php
+++ b/application/models/Admins_model.php
@@ -22,7 +22,7 @@ class Admins_model extends EA_Model {
     /**
      * @var array
      */
-    protected $casts = [
+    protected array $casts = [
         'id' => 'integer',
         'id_roles' => 'integer',
     ];
@@ -30,7 +30,7 @@ class Admins_model extends EA_Model {
     /**
      * @var array
      */
-    protected $api_resource = [
+    protected array $api_resource = [
         'id' => 'id',
         'firstName' => 'first_name',
         'lastName' => 'last_name',
@@ -55,6 +55,7 @@ class Admins_model extends EA_Model {
      * @return int Returns the admin ID.
      *
      * @throws InvalidArgumentException
+     * @throws Exception
      */
     public function save(array $admin): int
     {
@@ -153,7 +154,7 @@ class Admins_model extends EA_Model {
             ->where('roles.slug', DB_SLUG_ADMIN)
             ->where('users.email', $admin['email'])
             ->where('users.id !=', $admin_id)
-            ->where('users.delete_datetime', NULL)
+            ->where('users.delete_datetime')
             ->get()
             ->num_rows();
 
@@ -194,7 +195,7 @@ class Admins_model extends EA_Model {
      *
      * @return int Returns the admin ID.
      *
-     * @throws RuntimeException
+     * @throws RuntimeException|Exception
      */
     protected function insert(array $admin): int
     {
@@ -231,6 +232,7 @@ class Admins_model extends EA_Model {
      * @return int Returns the admin ID.
      *
      * @throws RuntimeException
+     * @throws Exception
      */
     protected function update(array $admin): int
     {
@@ -378,7 +380,7 @@ class Admins_model extends EA_Model {
     /**
      * Get all admins that match the provided criteria.
      *
-     * @param array|string $where Where conditions.
+     * @param array|string|null $where Where conditions.
      * @param int|null $limit Record limit.
      * @param int|null $offset Record offset.
      * @param string|null $order_by Order by.
@@ -386,7 +388,7 @@ class Admins_model extends EA_Model {
      *
      * @return array Returns an array of admins.
      */
-    public function get($where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
+    public function get(array|string $where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
     {
         $role_id = $this->get_admin_role_id();
 
@@ -474,9 +476,9 @@ class Admins_model extends EA_Model {
      *
      * @param int $admin_id Admin ID.
      * @param string $name Setting name.
-     * @param mixed $value Setting value.
+     * @param mixed|null $value Setting value.
      */
-    public function set_setting(int $admin_id, string $name, $value = NULL)
+    public function set_setting(int $admin_id, string $name, mixed $value = NULL)
     {
         if ( ! $this->db->update('user_settings', [$name => $value], ['id_users' => $admin_id]))
         {
diff --git a/application/models/Appointments_model.php b/application/models/Appointments_model.php
index 7519e0fe..9b84cc74 100644
--- a/application/models/Appointments_model.php
+++ b/application/models/Appointments_model.php
@@ -20,7 +20,7 @@ class Appointments_model extends EA_Model {
     /**
      * @var array
      */
-    protected $casts = [
+    protected array $casts = [
         'id' => 'integer',
         'is_unavailability' => 'boolean',
         'id_users_provider' => 'integer',
@@ -31,7 +31,7 @@ class Appointments_model extends EA_Model {
     /**
      * @var array
      */
-    protected $api_resource = [
+    protected array $api_resource = [
         'id' => 'id',
         'book' => 'book_datetime',
         'start' => 'start_datetime',
@@ -306,7 +306,7 @@ class Appointments_model extends EA_Model {
     /**
      * Get all appointments that match the provided criteria.
      *
-     * @param array|string $where Where conditions.
+     * @param array|string|null $where Where conditions.
      * @param int|null $limit Record limit.
      * @param int|null $offset Record offset.
      * @param string|null $order_by Order by.
@@ -314,7 +314,7 @@ class Appointments_model extends EA_Model {
      *
      * @return array Returns an array of appointments.
      */
-    public function get($where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
+    public function get(array|string $where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
     {
         if ($where !== NULL)
         {
diff --git a/application/models/Categories_model.php b/application/models/Categories_model.php
index 62adcd47..0505d568 100644
--- a/application/models/Categories_model.php
+++ b/application/models/Categories_model.php
@@ -22,14 +22,14 @@ class Categories_model extends EA_Model {
     /**
      * @var array
      */
-    protected $casts = [
+    protected array $casts = [
         'id' => 'integer',
     ];
 
     /**
      * @var array
      */
-    protected $api_resource = [
+    protected array $api_resource = [
         'id' => 'id',
         'name' => 'name',
         'description' => 'description',
@@ -225,7 +225,7 @@ class Categories_model extends EA_Model {
     /**
      * Get all services that match the provided criteria.
      *
-     * @param array|string $where Where conditions
+     * @param array|string|null $where Where conditions
      * @param int|null $limit Record limit.
      * @param int|null $offset Record offset.
      * @param string|null $order_by Order by.
@@ -233,7 +233,7 @@ class Categories_model extends EA_Model {
      *
      * @return array Returns an array of service categories.
      */
-    public function get($where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
+    public function get(array|string $where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
     {
         if ($where !== NULL)
         {
diff --git a/application/models/Consents_model.php b/application/models/Consents_model.php
index eb41cd3d..7800a6bf 100644
--- a/application/models/Consents_model.php
+++ b/application/models/Consents_model.php
@@ -22,7 +22,7 @@ class Consents_model extends EA_Model {
     /**
      * @var array
      */
-    protected $casts = [
+    protected array $casts = [
         'id' => 'integer',
     ];
 
@@ -130,7 +130,7 @@ class Consents_model extends EA_Model {
         }
         else
         {
-            return $this->db->update('consents', ['delete_datetime' => date('Y-m-d H:i:s')], ['id' => $consent_id]);
+            $this->db->update('consents', ['delete_datetime' => date('Y-m-d H:i:s')], ['id' => $consent_id]);
         }
     }
 
@@ -207,7 +207,7 @@ class Consents_model extends EA_Model {
     /**
      * Get all consents that match the provided criteria.
      *
-     * @param array|string $where Where conditions.
+     * @param array|string|null $where Where conditions.
      * @param int|null $limit Record limit.
      * @param int|null $offset Record offset.
      * @param string|null $order_by Order by.
@@ -215,7 +215,7 @@ class Consents_model extends EA_Model {
      *
      * @return array Returns an array of consents.
      */
-    public function get($where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
+    public function get(array|string $where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
     {
         if ($where !== NULL)
         {
diff --git a/application/models/Customers_model.php b/application/models/Customers_model.php
index d9c9bf4d..f00e1905 100644
--- a/application/models/Customers_model.php
+++ b/application/models/Customers_model.php
@@ -22,7 +22,7 @@ class Customers_model extends EA_Model {
     /**
      * @var array
      */
-    protected $casts = [
+    protected array $casts = [
         'id' => 'integer',
         'id_roles' => 'integer',
     ];
@@ -30,7 +30,7 @@ class Customers_model extends EA_Model {
     /**
      * @var array
      */
-    protected $api_resource = [
+    protected array $api_resource = [
         'id' => 'id',
         'firstName' => 'first_name',
         'lastName' => 'last_name',
@@ -133,7 +133,7 @@ class Customers_model extends EA_Model {
                 ->select()
                 ->from('users')
                 ->join('roles', 'roles.id = users.id_roles', 'inner')
-                ->where('users.delete_datetime', NULL)
+                ->where('users.delete_datetime')
                 ->where('roles.slug', DB_SLUG_CUSTOMER)
                 ->where('users.email', $customer['email'])
                 ->where('users.id !=', $customer_id)
@@ -285,7 +285,7 @@ class Customers_model extends EA_Model {
     /**
      * Get all customers that match the provided criteria.
      *
-     * @param array|string $where Where conditions.
+     * @param array|string|null $where Where conditions.
      * @param int|null $limit Record limit.
      * @param int|null $offset Record offset.
      * @param string|null $order_by Order by.
@@ -293,7 +293,7 @@ class Customers_model extends EA_Model {
      *
      * @return array Returns an array of customers.
      */
-    public function get($where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
+    public function get(array|string $where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
     {
         $role_id = $this->get_customer_role_id();
 
@@ -361,7 +361,7 @@ class Customers_model extends EA_Model {
             ->from('users')
             ->join('roles', 'roles.id = users.id_roles', 'inner')
             ->where('users.email', $customer['email'])
-            ->where('users.delete_datetime', NULL)
+            ->where('users.delete_datetime')
             ->where('roles.slug', DB_SLUG_CUSTOMER)
             ->get()
             ->num_rows();
@@ -391,7 +391,7 @@ class Customers_model extends EA_Model {
             ->from('users')
             ->join('roles', 'roles.id = users.id_roles', 'inner')
             ->where('users.email', $customer['email'])
-            ->where('users.delete_datetime', NULL)
+            ->where('users.delete_datetime')
             ->where('roles.slug', DB_SLUG_CUSTOMER)
             ->get()
             ->row_array();
diff --git a/application/models/Providers_model.php b/application/models/Providers_model.php
index 21442dc4..8d2c59b4 100755
--- a/application/models/Providers_model.php
+++ b/application/models/Providers_model.php
@@ -22,7 +22,7 @@ class Providers_model extends EA_Model {
     /**
      * @var array
      */
-    protected $casts = [
+    protected array $casts = [
         'id' => 'integer',
         'is_private' => 'boolean',
         'id_roles' => 'integer',
@@ -31,7 +31,7 @@ class Providers_model extends EA_Model {
     /**
      * @var array
      */
-    protected $api_resource = [
+    protected array $api_resource = [
         'id' => 'id',
         'firstName' => 'first_name',
         'lastName' => 'last_name',
@@ -57,6 +57,7 @@ class Providers_model extends EA_Model {
      * @return int Returns the provider ID.
      *
      * @throws InvalidArgumentException
+     * @throws Exception
      */
     public function save(array $provider): int
     {
@@ -168,7 +169,7 @@ class Providers_model extends EA_Model {
             ->where('roles.slug', DB_SLUG_PROVIDER)
             ->where('users.email', $provider['email'])
             ->where('users.id !=', $provider_id)
-            ->where('users.delete_datetime', NULL)
+            ->where('users.delete_datetime')
             ->get()
             ->num_rows();
 
@@ -209,7 +210,7 @@ class Providers_model extends EA_Model {
      *
      * @return int Returns the provider ID.
      *
-     * @throws RuntimeException
+     * @throws RuntimeException|Exception
      */
     protected function insert(array $provider): int
     {
@@ -248,7 +249,7 @@ class Providers_model extends EA_Model {
      *
      * @return int Returns the provider ID.
      *
-     * @throws RuntimeException
+     * @throws RuntimeException|Exception
      */
     protected function update(array $provider): int
     {
@@ -398,7 +399,7 @@ class Providers_model extends EA_Model {
     /**
      * Get all providers that match the provided criteria.
      *
-     * @param array|string $where Where conditions
+     * @param array|string|null $where Where conditions
      * @param int|null $limit Record limit.
      * @param int|null $offset Record offset.
      * @param string|null $order_by Order by.
@@ -406,7 +407,7 @@ class Providers_model extends EA_Model {
      *
      * @return array Returns an array of providers.
      */
-    public function get($where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
+    public function get(array|string $where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
     {
         $role_id = $this->get_provider_role_id();
 
@@ -519,9 +520,9 @@ class Providers_model extends EA_Model {
      *
      * @param int $provider_id Provider ID.
      * @param string $name Setting name.
-     * @param mixed $value Setting value.
+     * @param mixed|null $value Setting value.
      */
-    public function set_setting(int $provider_id, string $name, $value = NULL)
+    public function set_setting(int $provider_id, string $name, mixed $value = NULL)
     {
         if ( ! $this->db->update('user_settings', [$name => $value], ['id_users' => $provider_id]))
         {
@@ -579,6 +580,7 @@ class Providers_model extends EA_Model {
      * @param array $working_plan_exception Associative array with the working plan exception data.
      *
      * @throws InvalidArgumentException
+     * @throws Exception
      */
     public function save_working_plan_exception(int $provider_id, string $date, array $working_plan_exception)
     {
@@ -793,22 +795,18 @@ class Providers_model extends EA_Model {
 
         foreach ($resources as $resource)
         {
-            switch ($resource)
+            $provider['services'] = match ($resource)
             {
-                case 'services':
-                    $provider['services'] = $this
-                        ->db
-                        ->select('services.*')
-                        ->from('services')
-                        ->join('services_providers', 'services_providers.id_services = services.id', 'inner')
-                        ->where('id_users', $provider['id'])
-                        ->get()
-                        ->result_array();
-                    break;
-
-                default:
-                    throw new InvalidArgumentException('The requested provider relation is not supported: ' . $resource);
-            }
+                'services' => $this
+                    ->db
+                    ->select('services.*')
+                    ->from('services')
+                    ->join('services_providers', 'services_providers.id_services = services.id', 'inner')
+                    ->where('id_users', $provider['id'])
+                    ->get()
+                    ->result_array(),
+                default => throw new InvalidArgumentException('The requested provider relation is not supported: ' . $resource),
+            };
         }
     }
 
diff --git a/application/models/Roles_model.php b/application/models/Roles_model.php
index 525fc4ad..405995f7 100644
--- a/application/models/Roles_model.php
+++ b/application/models/Roles_model.php
@@ -22,7 +22,7 @@ class Roles_model extends EA_Model {
     /**
      * @var array
      */
-    protected $casts = [
+    protected array $casts = [
         'id' => 'integer',
         'is_admin' => 'boolean',
         'appointments' => 'integer',
@@ -223,7 +223,7 @@ class Roles_model extends EA_Model {
     /**
      * Get all roles that match the provided criteria.
      *
-     * @param array|string $where Where conditions
+     * @param array|string|null $where Where conditions
      * @param int|null $limit Record limit.
      * @param int|null $offset Record offset.
      * @param string|null $order_by Order by.
@@ -231,7 +231,7 @@ class Roles_model extends EA_Model {
      *
      * @return array Returns an array of roles.
      */
-    public function get($where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
+    public function get(array|string $where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
     {
         if ($where !== NULL)
         {
diff --git a/application/models/Secretaries_model.php b/application/models/Secretaries_model.php
index 5a562c2d..4c0ba22b 100644
--- a/application/models/Secretaries_model.php
+++ b/application/models/Secretaries_model.php
@@ -22,7 +22,7 @@ class Secretaries_model extends EA_Model {
     /**
      * @var array
      */
-    protected $casts = [
+    protected array $casts = [
         'id' => 'integer',
         'id_roles' => 'integer',
     ];
@@ -30,7 +30,7 @@ class Secretaries_model extends EA_Model {
     /**
      * @var array
      */
-    protected $api_resource = [
+    protected array $api_resource = [
         'id' => 'id',
         'firstName' => 'first_name',
         'lastName' => 'last_name',
@@ -55,6 +55,7 @@ class Secretaries_model extends EA_Model {
      * @return int Returns the secretary ID.
      *
      * @throws InvalidArgumentException
+     * @throws Exception
      */
     public function save(array $secretary): int
     {
@@ -166,7 +167,7 @@ class Secretaries_model extends EA_Model {
             ->where('roles.slug', DB_SLUG_SECRETARY)
             ->where('users.email', $secretary['email'])
             ->where('users.id !=', $secretary_id)
-            ->where('users.delete_datetime', NULL)
+            ->where('users.delete_datetime')
             ->get()
             ->num_rows();
 
@@ -207,7 +208,7 @@ class Secretaries_model extends EA_Model {
      *
      * @return int Returns the secretary ID.
      *
-     * @throws RuntimeException
+     * @throws RuntimeException|Exception
      */
     protected function insert(array $secretary): int
     {
@@ -247,7 +248,7 @@ class Secretaries_model extends EA_Model {
      *
      * @return int Returns the secretary ID.
      *
-     * @throws RuntimeException
+     * @throws RuntimeException|Exception
      */
     protected function update(array $secretary): int
     {
@@ -393,7 +394,7 @@ class Secretaries_model extends EA_Model {
     /**
      * Get all secretaries that match the provided criteria.
      *
-     * @param array|string $where Where conditions
+     * @param array|string|null $where Where conditions
      * @param int|null $limit Record limit.
      * @param int|null $offset Record offset.
      * @param string|null $order_by Order by.
@@ -401,7 +402,7 @@ class Secretaries_model extends EA_Model {
      *
      * @return array Returns an array of secretaries.
      */
-    public function get($where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
+    public function get(array|string $where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
     {
         $role_id = $this->get_secretary_role_id();
 
@@ -496,9 +497,9 @@ class Secretaries_model extends EA_Model {
      *
      * @param int $secretary_id Secretary ID.
      * @param string $name Setting name.
-     * @param mixed $value Setting value.
+     * @param mixed|null $value Setting value.
      */
-    public function set_setting(int $secretary_id, string $name, $value = NULL)
+    public function set_setting(int $secretary_id, string $name, mixed $value = NULL)
     {
         if ( ! $this->db->update('user_settings', [$name => $value], ['id_users' => $secretary_id]))
         {
@@ -644,22 +645,18 @@ class Secretaries_model extends EA_Model {
 
         foreach ($resources as $resource)
         {
-            switch ($resource)
+            $secretary['providers'] = match ($resource)
             {
-                case 'providers':
-                    $secretary['providers'] = $this
-                        ->db
-                        ->select('users.*')
-                        ->from('users')
-                        ->join('secretaries_providers', 'secretaries_providers.id_users_provider = users.id', 'inner')
-                        ->where('id_users_secretary', $secretary['id'])
-                        ->get()
-                        ->result_array();
-                    break;
-
-                default:
-                    throw new InvalidArgumentException('The requested secretary relation is not supported: ' . $resource);
-            }
+                'providers' => $this
+                    ->db
+                    ->select('users.*')
+                    ->from('users')
+                    ->join('secretaries_providers', 'secretaries_providers.id_users_provider = users.id', 'inner')
+                    ->where('id_users_secretary', $secretary['id'])
+                    ->get()
+                    ->result_array(),
+                default => throw new InvalidArgumentException('The requested secretary relation is not supported: ' . $resource),
+            };
         }
     }
 
diff --git a/application/models/Services_model.php b/application/models/Services_model.php
index a66851ae..a59c83fe 100644
--- a/application/models/Services_model.php
+++ b/application/models/Services_model.php
@@ -22,7 +22,7 @@ class Services_model extends EA_Model {
     /**
      * @var array
      */
-    protected $casts = [
+    protected array $casts = [
         'id' => 'integer',
         'price' => 'float',
         'attendants_number' => 'integer',
@@ -33,7 +33,7 @@ class Services_model extends EA_Model {
     /**
      * @var array
      */
-    protected $api_resource = [
+    protected array $api_resource = [
         'id' => 'id',
         'name' => 'name',
         'duration' => 'duration',
@@ -281,7 +281,7 @@ class Services_model extends EA_Model {
     /**
      * Get all services that match the provided criteria.
      *
-     * @param array|string $where Where conditions
+     * @param array|string|null $where Where conditions
      * @param int|null $limit Record limit.
      * @param int|null $offset Record offset.
      * @param string|null $order_by Order by.
@@ -289,7 +289,7 @@ class Services_model extends EA_Model {
      *
      * @return array Returns an array of services.
      */
-    public function get($where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
+    public function get(array|string $where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
     {
         if ($where !== NULL)
         {
@@ -417,20 +417,16 @@ class Services_model extends EA_Model {
 
         foreach ($resources as $resource)
         {
-            switch ($resource)
+            $service['category'] = match ($resource)
             {
-                case 'category':
-                    $service['category'] = $this
-                        ->db
-                        ->get_where('categories', [
-                            'id' => $service['id_categories'] ?? $service['categoryId'] ?? NULL
-                        ])
-                        ->row_array();
-                    break;
-
-                default:
-                    throw new InvalidArgumentException('The requested appointment relation is not supported: ' . $resource);
-            }
+                'category' => $this
+                    ->db
+                    ->get_where('categories', [
+                        'id' => $service['id_categories'] ?? $service['categoryId'] ?? NULL
+                    ])
+                    ->row_array(),
+                default => throw new InvalidArgumentException('The requested appointment relation is not supported: ' . $resource),
+            };
         }
     }
 
diff --git a/application/models/Settings_model.php b/application/models/Settings_model.php
index bf56ea73..3511481e 100644
--- a/application/models/Settings_model.php
+++ b/application/models/Settings_model.php
@@ -22,14 +22,14 @@ class Settings_model extends EA_Model {
     /**
      * @var array
      */
-    protected $casts = [
+    protected array $casts = [
         'id' => 'integer',
     ];
 
     /**
      * @var array 
      */
-    protected $api_resource = [
+    protected array $api_resource = [
         'name' => 'name',  
         'value' => 'value',  
     ];
@@ -224,7 +224,7 @@ class Settings_model extends EA_Model {
     /**
      * Get all settings that match the provided criteria.
      *
-     * @param array|string $where Where conditions
+     * @param array|string|null $where Where conditions
      * @param int|null $limit Record limit.
      * @param int|null $offset Record offset.
      * @param string|null $order_by Order by.
@@ -232,7 +232,7 @@ class Settings_model extends EA_Model {
      * 
      * @return array Returns an array of settings.
      */
-    public function get($where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
+    public function get(array|string $where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
     {
         if ($where !== NULL)
         {
diff --git a/application/models/Unavailabilities_model.php b/application/models/Unavailabilities_model.php
index aa139816..730967fc 100644
--- a/application/models/Unavailabilities_model.php
+++ b/application/models/Unavailabilities_model.php
@@ -20,7 +20,7 @@ class Unavailabilities_model extends EA_Model {
     /**
      * @var array
      */
-    protected $casts = [
+    protected array $casts = [
         'id' => 'integer',
         'is_unavailability' => 'boolean',
         'id_users_provider' => 'integer',
@@ -31,7 +31,7 @@ class Unavailabilities_model extends EA_Model {
     /**
      * @var array
      */
-    protected $api_resource = [
+    protected array $api_resource = [
         'id' => 'id',
         'book' => 'book_datetime',
         'start' => 'start_datetime',
@@ -275,7 +275,7 @@ class Unavailabilities_model extends EA_Model {
     /**
      * Get all unavailabilities that match the provided criteria.
      *
-     * @param array|string $where Where conditions.
+     * @param array|string|null $where Where conditions.
      * @param int|null $limit Record limit.
      * @param int|null $offset Record offset.
      * @param string|null $order_by Order by.
@@ -283,7 +283,7 @@ class Unavailabilities_model extends EA_Model {
      *
      * @return array Returns an array of unavailabilities.
      */
-    public function get($where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
+    public function get(array|string $where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
     {
         if ($where !== NULL)
         {
@@ -386,20 +386,16 @@ class Unavailabilities_model extends EA_Model {
 
         foreach ($resources as $resource)
         {
-            switch ($resource)
+            $unavailability['provider'] = match ($resource)
             {
-                case 'provider':
-                    $unavailability['provider'] = $this
-                        ->db
-                        ->get_where('users', [
-                            'id' => $unavailability['id_users_provider'] ?? $unavailability['providerId'] ?? NULL
-                        ])
-                        ->row_array();
-                    break;
-
-                default:
-                    throw new InvalidArgumentException('The requested unavailability relation is not supported: ' . $resource);
-            }
+                'provider' => $this
+                    ->db
+                    ->get_where('users', [
+                        'id' => $unavailability['id_users_provider'] ?? $unavailability['providerId'] ?? NULL
+                    ])
+                    ->row_array(),
+                default => throw new InvalidArgumentException('The requested unavailability relation is not supported: ' . $resource),
+            };
         }
     }
 
diff --git a/application/models/Users_model.php b/application/models/Users_model.php
index 7e3db0af..b2102f80 100644
--- a/application/models/Users_model.php
+++ b/application/models/Users_model.php
@@ -22,7 +22,7 @@ class Users_model extends EA_Model {
     /**
      * @var array
      */
-    protected $casts = [
+    protected array $casts = [
         'id' => 'integer',
         'id_roles' => 'integer',
     ];
@@ -30,7 +30,7 @@ class Users_model extends EA_Model {
     /**
      * @var array
      */
-    protected $api_resource = [
+    protected array $api_resource = [
         'id' => 'id',
         'firstName' => 'first_name',
         'lastName' => 'last_name',
@@ -55,6 +55,7 @@ class Users_model extends EA_Model {
      * @return int Returns the user ID.
      *
      * @throws InvalidArgumentException
+     * @throws Exception
      */
     public function save(array $user): int
     {
@@ -109,7 +110,7 @@ class Users_model extends EA_Model {
      *
      * @return int Returns the user ID.
      *
-     * @throws RuntimeException
+     * @throws RuntimeException|Exception
      */
     protected function insert(array $user): int
     {
@@ -140,7 +141,7 @@ class Users_model extends EA_Model {
      *
      * @return int Returns the user ID.
      *
-     * @throws RuntimeException
+     * @throws RuntimeException|Exception
      */
     protected function update(array $user): int
     {
@@ -274,7 +275,7 @@ class Users_model extends EA_Model {
     /**
      * Get all users that match the provided criteria.
      *
-     * @param array|string $where Where conditions
+     * @param array|string|null $where Where conditions
      * @param int|null $limit Record limit.
      * @param int|null $offset Record offset.
      * @param string|null $order_by Order by.
@@ -282,7 +283,7 @@ class Users_model extends EA_Model {
      * 
      * @return array Returns an array of users.
      */
-    public function get($where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
+    public function get(array|string $where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
     {
         if ($where !== NULL)
         {
diff --git a/application/models/Webhooks_model.php b/application/models/Webhooks_model.php
index 57dd9cb1..4dc30c03 100644
--- a/application/models/Webhooks_model.php
+++ b/application/models/Webhooks_model.php
@@ -22,7 +22,7 @@ class Webhooks_model extends EA_Model {
     /**
      * @var array
      */
-    protected $casts = [
+    protected array $casts = [
         'id' => 'integer',
         'is_active' => 'boolean',
         'is_ssl_verified' => 'boolean',
@@ -31,7 +31,7 @@ class Webhooks_model extends EA_Model {
     /**
      * @var array
      */
-    protected $api_resource = [
+    protected array $api_resource = [
         'id' => 'id',
         'name' => 'name',
         'url' => 'url',
@@ -222,7 +222,7 @@ class Webhooks_model extends EA_Model {
     /**
      * Get all webhooks that match the provided criteria.
      *
-     * @param array|string $where Where conditions.
+     * @param array|string|null $where Where conditions.
      * @param int|null $limit Record limit.
      * @param int|null $offset Record offset.
      * @param string|null $order_by Order by.
@@ -230,7 +230,7 @@ class Webhooks_model extends EA_Model {
      *
      * @return array Returns an array of webhooks.
      */
-    public function get($where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
+    public function get(array|string $where = NULL, int $limit = NULL, int $offset = NULL, string $order_by = NULL, bool $with_trashed = FALSE): array
     {
         if ($where !== NULL)
         {