diff --git a/application/controllers/About.php b/application/controllers/About.php
index e6580b6c..ba709f32 100644
--- a/application/controllers/About.php
+++ b/application/controllers/About.php
@@ -44,7 +44,7 @@ class About extends EA_Controller
     /**
      * Render the settings page.
      */
-    public function index()
+    public function index(): void
     {
         session(['dest_url' => site_url('about')]);
 
diff --git a/application/controllers/Account.php b/application/controllers/Account.php
index a69a9656..c13e953c 100644
--- a/application/controllers/Account.php
+++ b/application/controllers/Account.php
@@ -44,7 +44,7 @@ class Account extends EA_Controller
     /**
      * Render the settings page.
      */
-    public function index()
+    public function index(): void
     {
         session(['dest_url' => site_url('account')]);
 
@@ -79,7 +79,7 @@ class Account extends EA_Controller
     /**
      * Save general settings.
      */
-    public function save()
+    public function save(): void
     {
         try {
             if (cannot('edit', PRIV_USER_SETTINGS)) {
@@ -131,7 +131,7 @@ class Account extends EA_Controller
     /**
      * Make sure the username is valid and unique in the database.
      */
-    public function validate_username()
+    public function validate_username(): void
     {
         try {
             $username = request('username');
diff --git a/application/controllers/Admins.php b/application/controllers/Admins.php
index 7573acde..eea99651 100644
--- a/application/controllers/Admins.php
+++ b/application/controllers/Admins.php
@@ -41,7 +41,7 @@ class Admins extends EA_Controller
      * On this page admin users will be able to manage admins, which are eventually selected by customers during the
      * booking process.
      */
-    public function index()
+    public function index(): void
     {
         session(['dest_url' => site_url('admins')]);
 
@@ -82,7 +82,7 @@ class Admins extends EA_Controller
     /**
      * Filter admins by the provided keyword.
      */
-    public function search()
+    public function search(): void
     {
         try {
             if (cannot('view', PRIV_USERS)) {
@@ -108,7 +108,7 @@ class Admins extends EA_Controller
     /**
      * Store a new admin.
      */
-    public function store()
+    public function store(): void
     {
         try {
             if (cannot('add', PRIV_USERS)) {
@@ -153,7 +153,7 @@ class Admins extends EA_Controller
     /**
      * Find an admin.
      */
-    public function find()
+    public function find(): void
     {
         try {
             if (cannot('view', PRIV_USERS)) {
@@ -173,7 +173,7 @@ class Admins extends EA_Controller
     /**
      * Update an admin.
      */
-    public function update()
+    public function update(): void
     {
         try {
             if (cannot('edit', PRIV_USERS)) {
@@ -219,7 +219,7 @@ class Admins extends EA_Controller
     /**
      * Remove an admin.
      */
-    public function destroy()
+    public function destroy(): void
     {
         try {
             if (cannot('delete', PRIV_USERS)) {
diff --git a/application/controllers/Api_settings.php b/application/controllers/Api_settings.php
index 77b05adf..d46d9177 100644
--- a/application/controllers/Api_settings.php
+++ b/application/controllers/Api_settings.php
@@ -35,7 +35,7 @@ class Api_settings extends EA_Controller
     /**
      * Render the settings page.
      */
-    public function index()
+    public function index(): void
     {
         session(['dest_url' => site_url('api_settings')]);
 
@@ -71,7 +71,7 @@ class Api_settings extends EA_Controller
     /**
      * Save general settings.
      */
-    public function save()
+    public function save(): void
     {
         try {
             if (cannot('edit', PRIV_SYSTEM_SETTINGS)) {
diff --git a/application/controllers/Appointments.php b/application/controllers/Appointments.php
index fc566197..e4241b22 100644
--- a/application/controllers/Appointments.php
+++ b/application/controllers/Appointments.php
@@ -53,7 +53,7 @@ class Appointments extends EA_Controller
     /**
      * Filter appointments by the provided keyword.
      */
-    public function search()
+    public function search(): void
     {
         try {
             if (cannot('view', PRIV_APPOINTMENTS)) {
@@ -79,7 +79,7 @@ class Appointments extends EA_Controller
     /**
      * Store a new appointment.
      */
-    public function store()
+    public function store(): void
     {
         try {
             if (cannot('add', PRIV_APPOINTMENTS)) {
@@ -118,7 +118,7 @@ class Appointments extends EA_Controller
     /**
      * Find an appointment.
      */
-    public function find()
+    public function find(): void
     {
         try {
             if (cannot('view', PRIV_APPOINTMENTS)) {
@@ -138,7 +138,7 @@ class Appointments extends EA_Controller
     /**
      * Update a appointment.
      */
-    public function update()
+    public function update(): void
     {
         try {
             if (cannot('edit', PRIV_APPOINTMENTS)) {
@@ -174,7 +174,7 @@ class Appointments extends EA_Controller
     /**
      * Remove a appointment.
      */
-    public function destroy()
+    public function destroy(): void
     {
         try {
             if (cannot('delete', PRIV_APPOINTMENTS)) {
diff --git a/application/controllers/Backend.php b/application/controllers/Backend.php
index b7eb42bc..2065b515 100755
--- a/application/controllers/Backend.php
+++ b/application/controllers/Backend.php
@@ -46,7 +46,7 @@ class Backend extends EA_Controller
      *
      * @param string $appointment_hash Appointment edit dialog will appear when the page loads (default '').
      */
-    public function index(string $appointment_hash = '')
+    public function index(string $appointment_hash = ''): void
     {
         if (empty($appointment_hash)) {
             redirect('calendar');
@@ -58,7 +58,7 @@ class Backend extends EA_Controller
     /**
      * Display the customers page.
      */
-    public function customers()
+    public function customers(): void
     {
         redirect('customers');
     }
@@ -66,7 +66,7 @@ class Backend extends EA_Controller
     /**
      * Display the services page.
      */
-    public function services()
+    public function services(): void
     {
         redirect('services');
     }
@@ -77,7 +77,7 @@ class Backend extends EA_Controller
      * Notice: Since the "users" page is split into multiple pages (providers, secretaries, admins), this method will
      * redirect to "providers" page by default
      */
-    public function users()
+    public function users(): void
     {
         redirect('providers');
     }
@@ -88,7 +88,7 @@ class Backend extends EA_Controller
      * Notice: Since the "settings" page is split into multiple pages (general, business, booking etc), this method will
      * redirect to "general" page by default.
      */
-    public function settings()
+    public function settings(): void
     {
         redirect('general_settings');
     }
@@ -96,7 +96,7 @@ class Backend extends EA_Controller
     /**
      * Display the update page.
      */
-    public function update()
+    public function update(): void
     {
         redirect('update');
     }
diff --git a/application/controllers/Backend_api.php b/application/controllers/Backend_api.php
index b0798d19..af9dcbfd 100755
--- a/application/controllers/Backend_api.php
+++ b/application/controllers/Backend_api.php
@@ -44,7 +44,7 @@ class Backend_api extends EA_Controller
     /**
      * Get Calendar Events
      */
-    public function ajax_get_calendar_events()
+    public function ajax_get_calendar_events(): void
     {
         redirect('calendar/get_calendar_appointments_for_table_view');
     }
@@ -52,7 +52,7 @@ class Backend_api extends EA_Controller
     /**
      * Get the registered appointments for the given date period and record.
      */
-    public function ajax_get_calendar_appointments()
+    public function ajax_get_calendar_appointments(): void
     {
         redirect('calendar/get_calendar_appointments');
     }
@@ -60,7 +60,7 @@ class Backend_api extends EA_Controller
     /**
      * Save appointment changes that are made from the backend calendar page.
      */
-    public function ajax_save_appointment()
+    public function ajax_save_appointment(): void
     {
         redirect('calendar/save_appointment');
     }
@@ -68,7 +68,7 @@ class Backend_api extends EA_Controller
     /**
      * Delete appointment from the database.
      */
-    public function ajax_delete_appointment()
+    public function ajax_delete_appointment(): void
     {
         redirect('calendar/delete_appointment');
     }
@@ -76,7 +76,7 @@ class Backend_api extends EA_Controller
     /**
      * Disable a providers sync setting.
      */
-    public function ajax_disable_provider_sync()
+    public function ajax_disable_provider_sync(): void
     {
         redirect('google/disable_provider_sync');
     }
@@ -84,7 +84,7 @@ class Backend_api extends EA_Controller
     /**
      * Filter the customer records with the given key string.
      */
-    public function ajax_filter_customers()
+    public function ajax_filter_customers(): void
     {
         redirect('customers/search');
     }
@@ -92,7 +92,7 @@ class Backend_api extends EA_Controller
     /**
      * Insert or update an unavailability.
      */
-    public function ajax_save_unavailability()
+    public function ajax_save_unavailability(): void
     {
         redirect('calendar/save_unavailability');
     }
@@ -100,7 +100,7 @@ class Backend_api extends EA_Controller
     /**
      * Delete an unavailability time period from database.
      */
-    public function ajax_delete_unavailability()
+    public function ajax_delete_unavailability(): void
     {
         redirect('calendar/delete_unavailability');
     }
@@ -108,7 +108,7 @@ class Backend_api extends EA_Controller
     /**
      * Insert of update working plan exceptions to database.
      */
-    public function ajax_save_working_plan_exception()
+    public function ajax_save_working_plan_exception(): void
     {
         redirect('calendar/save_working_plan_exception');
     }
@@ -116,7 +116,7 @@ class Backend_api extends EA_Controller
     /**
      * Delete a working plan exceptions time period to database.
      */
-    public function ajax_delete_working_plan_exception()
+    public function ajax_delete_working_plan_exception(): void
     {
         redirect('calendar/delete_working_plan_exception');
     }
@@ -124,7 +124,7 @@ class Backend_api extends EA_Controller
     /**
      * Save (insert or update) a customer record.
      */
-    public function ajax_save_customer()
+    public function ajax_save_customer(): void
     {
         redirect('customers/create'); // or "customers/update"
     }
@@ -132,7 +132,7 @@ class Backend_api extends EA_Controller
     /**
      * Delete customer from database.
      */
-    public function ajax_delete_customer()
+    public function ajax_delete_customer(): void
     {
         redirect('customers/destroy');
     }
@@ -140,7 +140,7 @@ class Backend_api extends EA_Controller
     /**
      * Save (insert or update) service record.
      */
-    public function ajax_save_service()
+    public function ajax_save_service(): void
     {
         redirect('services/create'); // or "services/update"
     }
@@ -148,7 +148,7 @@ class Backend_api extends EA_Controller
     /**
      * Delete service record from database.
      */
-    public function ajax_delete_service()
+    public function ajax_delete_service(): void
     {
         redirect('services/destroy');
     }
@@ -156,7 +156,7 @@ class Backend_api extends EA_Controller
     /**
      * Filter service records by given key string.
      */
-    public function ajax_filter_services()
+    public function ajax_filter_services(): void
     {
         redirect('services/search');
     }
@@ -164,7 +164,7 @@ class Backend_api extends EA_Controller
     /**
      * Save (insert or update) category record.
      */
-    public function ajax_save_service_category()
+    public function ajax_save_service_category(): void
     {
         redirect('categories/create'); // or "categories/update"
     }
@@ -172,7 +172,7 @@ class Backend_api extends EA_Controller
     /**
      * Delete category record from database.
      */
-    public function ajax_delete_service_category()
+    public function ajax_delete_service_category(): void
     {
         redirect('categories/destroy');
     }
@@ -180,7 +180,7 @@ class Backend_api extends EA_Controller
     /**
      * Filter services categories with key string.
      */
-    public function ajax_filter_service_categories()
+    public function ajax_filter_service_categories(): void
     {
         redirect('categories/search');
     }
@@ -188,7 +188,7 @@ class Backend_api extends EA_Controller
     /**
      * Filter admin records with string key.
      */
-    public function ajax_filter_admins()
+    public function ajax_filter_admins(): void
     {
         redirect('admins/search');
     }
@@ -196,7 +196,7 @@ class Backend_api extends EA_Controller
     /**
      * Save (insert or update) admin record into database.
      */
-    public function ajax_save_admin()
+    public function ajax_save_admin(): void
     {
         redirect('admins/create'); // or "admins/update"
     }
@@ -204,7 +204,7 @@ class Backend_api extends EA_Controller
     /**
      * Delete an admin record from the database.
      */
-    public function ajax_delete_admin()
+    public function ajax_delete_admin(): void
     {
         redirect('admins/destroy');
     }
@@ -212,7 +212,7 @@ class Backend_api extends EA_Controller
     /**
      * Filter provider records with string key.
      */
-    public function ajax_filter_providers()
+    public function ajax_filter_providers(): void
     {
         redirect('providers/search');
     }
@@ -220,7 +220,7 @@ class Backend_api extends EA_Controller
     /**
      * Save (insert or update) a provider record into database.
      */
-    public function ajax_save_provider()
+    public function ajax_save_provider(): void
     {
         redirect('providers/create'); // or "providers/update"
     }
@@ -228,7 +228,7 @@ class Backend_api extends EA_Controller
     /**
      * Delete a provider record from the database.
      */
-    public function ajax_delete_provider()
+    public function ajax_delete_provider(): void
     {
         redirect('providers/destroy');
     }
@@ -236,7 +236,7 @@ class Backend_api extends EA_Controller
     /**
      * Filter secretary records with string key.
      */
-    public function ajax_filter_secretaries()
+    public function ajax_filter_secretaries(): void
     {
         redirect('secretaries/search');
     }
@@ -244,7 +244,7 @@ class Backend_api extends EA_Controller
     /**
      * Save (insert or update) a secretary record into database.
      */
-    public function ajax_save_secretary()
+    public function ajax_save_secretary(): void
     {
         redirect('secretaries/create'); // or "secretaries/update"
     }
@@ -252,7 +252,7 @@ class Backend_api extends EA_Controller
     /**
      * Delete a secretary record from the database.
      */
-    public function ajax_delete_secretary()
+    public function ajax_delete_secretary(): void
     {
         redirect('secretaries/destroy');
     }
@@ -260,7 +260,7 @@ class Backend_api extends EA_Controller
     /**
      * Save a setting or multiple settings in the database.
      */
-    public function ajax_save_settings()
+    public function ajax_save_settings(): void
     {
         redirect('general_settings/save'); // or "business_settings/save", "booking_settings/save", "legal_settings/save"
     }
@@ -268,7 +268,7 @@ class Backend_api extends EA_Controller
     /**
      * This method checks whether the username already exists in the database.
      */
-    public function ajax_validate_username()
+    public function ajax_validate_username(): void
     {
         redirect('account/validate_username');
     }
@@ -276,7 +276,7 @@ class Backend_api extends EA_Controller
     /**
      * Change system language for current user.
      */
-    public function ajax_change_language()
+    public function ajax_change_language(): void
     {
         redirect('account/change_language');
     }
@@ -284,7 +284,7 @@ class Backend_api extends EA_Controller
     /**
      * This method will return a list of the available Google Calendars.
      */
-    public function ajax_get_google_calendars()
+    public function ajax_get_google_calendars(): void
     {
         redirect('google/get_google_calendars');
     }
@@ -292,7 +292,7 @@ class Backend_api extends EA_Controller
     /**
      * Select a specific google calendar for a provider.
      */
-    public function ajax_select_google_calendar()
+    public function ajax_select_google_calendar(): void
     {
         redirect('google/select_google_calendar');
     }
@@ -300,7 +300,7 @@ class Backend_api extends EA_Controller
     /**
      * Apply global working plan to all providers.
      */
-    public function ajax_apply_global_working_plan()
+    public function ajax_apply_global_working_plan(): void
     {
         redirect('business_settings/apply_global_working_plan');
     }
diff --git a/application/controllers/Blocked_periods.php b/application/controllers/Blocked_periods.php
index 3a8ca58c..f56a4f21 100644
--- a/application/controllers/Blocked_periods.php
+++ b/application/controllers/Blocked_periods.php
@@ -41,7 +41,7 @@ class Blocked_periods extends EA_Controller
      * On this page admin users will be able to manage blocked-periods, which are eventually selected by customers during the
      * booking process.
      */
-    public function index()
+    public function index(): void
     {
         session(['dest_url' => site_url('blocked_periods')]);
 
@@ -81,7 +81,7 @@ class Blocked_periods extends EA_Controller
     /**
      * Filter blocked-periods by the provided keyword.
      */
-    public function search()
+    public function search(): void
     {
         try {
             if (cannot('view', PRIV_BLOCKED_PERIODS)) {
@@ -107,7 +107,7 @@ class Blocked_periods extends EA_Controller
     /**
      * Store a new service-category.
      */
-    public function store()
+    public function store(): void
     {
         try {
             if (cannot('add', PRIV_BLOCKED_PERIODS)) {
@@ -136,7 +136,7 @@ class Blocked_periods extends EA_Controller
     /**
      * Find a service-category.
      */
-    public function find()
+    public function find(): void
     {
         try {
             if (cannot('view', PRIV_BLOCKED_PERIODS)) {
@@ -156,7 +156,7 @@ class Blocked_periods extends EA_Controller
     /**
      * Update a service-category.
      */
-    public function update()
+    public function update(): void
     {
         try {
             if (cannot('edit', PRIV_BLOCKED_PERIODS)) {
@@ -191,7 +191,7 @@ class Blocked_periods extends EA_Controller
     /**
      * Remove a service-category.
      */
-    public function destroy()
+    public function destroy(): void
     {
         try {
             if (cannot('delete', PRIV_BLOCKED_PERIODS)) {
diff --git a/application/controllers/Booking.php b/application/controllers/Booking.php
index e23597bb..cc0fae4b 100755
--- a/application/controllers/Booking.php
+++ b/application/controllers/Booking.php
@@ -54,7 +54,7 @@ class Booking extends EA_Controller
      *
      * @param string $appointment_hash
      */
-    public function reschedule(string $appointment_hash)
+    public function reschedule(string $appointment_hash): void
     {
         html_vars(['appointment_hash' => $appointment_hash]);
 
@@ -66,7 +66,7 @@ class Booking extends EA_Controller
      *
      * This method creates the appointment book wizard.
      */
-    public function index()
+    public function index(): void
     {
         if (!is_app_installed()) {
             redirect('installation');
@@ -289,7 +289,7 @@ class Booking extends EA_Controller
     /**
      * Register the appointment to the database.
      */
-    public function register()
+    public function register(): void
     {
         try {
             $disable_booking = setting('disable_booking');
@@ -584,7 +584,7 @@ class Booking extends EA_Controller
      * This method answers to an AJAX request. It calculates the available hours for the given service, provider and
      * date.
      */
-    public function get_available_hours()
+    public function get_available_hours(): void
     {
         try {
             $disable_booking = setting('disable_booking');
@@ -666,7 +666,7 @@ class Booking extends EA_Controller
      *
      * Outputs a JSON string with the unavailability dates. that are unavailability.
      */
-    public function get_unavailable_dates()
+    public function get_unavailable_dates(): void
     {
         try {
             $disable_booking = setting('disable_booking');
diff --git a/application/controllers/Booking_cancellation.php b/application/controllers/Booking_cancellation.php
index 950ac461..d5ec7647 100755
--- a/application/controllers/Booking_cancellation.php
+++ b/application/controllers/Booking_cancellation.php
@@ -46,7 +46,7 @@ class Booking_cancellation extends EA_Controller
      *
      * @param string $appointment_hash This appointment hash identifier.
      */
-    public function of(string $appointment_hash)
+    public function of(string $appointment_hash): void
     {
         try {
             $disable_booking = setting('disable_booking');
diff --git a/application/controllers/Booking_confirmation.php b/application/controllers/Booking_confirmation.php
index 7e6fb689..d743678f 100755
--- a/application/controllers/Booking_confirmation.php
+++ b/application/controllers/Booking_confirmation.php
@@ -40,7 +40,7 @@ class Booking_confirmation extends EA_Controller
      *
      * @throws Exception
      */
-    public function of()
+    public function of(): void
     {
         $appointment_hash = $this->uri->segment(3);
 
diff --git a/application/controllers/Booking_settings.php b/application/controllers/Booking_settings.php
index c3901cdb..15c5f88b 100644
--- a/application/controllers/Booking_settings.php
+++ b/application/controllers/Booking_settings.php
@@ -44,7 +44,7 @@ class Booking_settings extends EA_Controller
     /**
      * Render the settings page.
      */
-    public function index()
+    public function index(): void
     {
         session(['dest_url' => site_url('booking_settings')]);
 
@@ -80,7 +80,7 @@ class Booking_settings extends EA_Controller
     /**
      * Save booking settings.
      */
-    public function save()
+    public function save(): void
     {
         try {
             if (cannot('edit', PRIV_SYSTEM_SETTINGS)) {
diff --git a/application/controllers/Business_settings.php b/application/controllers/Business_settings.php
index 55b0e45a..cef2806b 100644
--- a/application/controllers/Business_settings.php
+++ b/application/controllers/Business_settings.php
@@ -44,7 +44,7 @@ class Business_settings extends EA_Controller
     /**
      * Render the settings page.
      */
-    public function index()
+    public function index(): void
     {
         session(['dest_url' => site_url('business_settings')]);
 
@@ -82,7 +82,7 @@ class Business_settings extends EA_Controller
     /**
      * Save general settings.
      */
-    public function save()
+    public function save(): void
     {
         try {
             if (cannot('edit', PRIV_SYSTEM_SETTINGS)) {
diff --git a/application/controllers/Captcha.php b/application/controllers/Captcha.php
index 31b69a39..467bbbc2 100644
--- a/application/controllers/Captcha.php
+++ b/application/controllers/Captcha.php
@@ -33,7 +33,7 @@ class Captcha extends EA_Controller
     /**
      * Make a request to this method to get a captcha image.
      */
-    public function index()
+    public function index(): void
     {
         $this->captcha_builder->setDistortion(true);
         $this->captcha_builder->setMaxBehindLines(1);
diff --git a/application/controllers/Consents.php b/application/controllers/Consents.php
index 3104deb0..bc8c3b3e 100644
--- a/application/controllers/Consents.php
+++ b/application/controllers/Consents.php
@@ -33,7 +33,7 @@ class Consents extends EA_Controller
     /**
      * Save (insert or update) the consent
      */
-    public function save()
+    public function save(): void
     {
         try {
             $consent = request('consent');
diff --git a/application/controllers/Console.php b/application/controllers/Console.php
index a81ad3d0..ce991d87 100644
--- a/application/controllers/Console.php
+++ b/application/controllers/Console.php
@@ -50,8 +50,10 @@ class Console extends EA_Controller
      * Usage:
      *
      * php index.php console install
+     *
+     * @throws Exception
      */
-    public function install()
+    public function install(): void
     {
         $this->instance->migrate('fresh');
 
@@ -70,7 +72,7 @@ class Console extends EA_Controller
      * Notice:
      *
      * Do not use this method to install the app as it will not seed the database with the initial entries (admin,
-     * provider, service, settings etc).
+     * provider, service, settings etc.).
      *
      * Usage:
      *
@@ -80,7 +82,7 @@ class Console extends EA_Controller
      *
      * @param string $type
      */
-    public function migrate(string $type = '')
+    public function migrate(string $type = ''): void
     {
         $this->instance->migrate($type);
     }
@@ -93,8 +95,9 @@ class Console extends EA_Controller
      * Usage:
      *
      * php index.php console seed
+     * @throws Exception
      */
-    public function seed()
+    public function seed(): void
     {
         $this->instance->seed();
     }
@@ -112,7 +115,7 @@ class Console extends EA_Controller
      *
      * @throws Exception
      */
-    public function backup()
+    public function backup(): void
     {
         $this->instance->backup($GLOBALS['argv'][3] ?? null);
     }
@@ -130,7 +133,7 @@ class Console extends EA_Controller
      *
      * php index.php console sync
      */
-    public function sync()
+    public function sync(): void
     {
         $providers = $this->providers_model->get();
 
@@ -152,7 +155,7 @@ class Console extends EA_Controller
      *
      * php index.php console help
      */
-    public function help()
+    public function help(): void
     {
         $help = [
             '',
diff --git a/application/controllers/Customers.php b/application/controllers/Customers.php
index 134dfd83..2985d270 100644
--- a/application/controllers/Customers.php
+++ b/application/controllers/Customers.php
@@ -44,7 +44,7 @@ class Customers extends EA_Controller
      * On this page admin users will be able to manage customers, which are eventually selected by customers during the
      * booking process.
      */
-    public function index()
+    public function index(): void
     {
         session(['dest_url' => site_url('customers')]);
 
@@ -114,7 +114,7 @@ class Customers extends EA_Controller
     /**
      * Find a customer.
      */
-    public function find()
+    public function find(): void
     {
         try {
             if (cannot('view', PRIV_CUSTOMERS)) {
@@ -140,7 +140,7 @@ class Customers extends EA_Controller
     /**
      * Filter customers by the provided keyword.
      */
-    public function search()
+    public function search(): void
     {
         try {
             if (cannot('view', PRIV_CUSTOMERS)) {
@@ -184,7 +184,7 @@ class Customers extends EA_Controller
     /**
      * Store a new customer.
      */
-    public function store()
+    public function store(): void
     {
         try {
             if (cannot('add', PRIV_CUSTOMERS)) {
@@ -234,7 +234,7 @@ class Customers extends EA_Controller
     /**
      * Update a customer.
      */
-    public function update()
+    public function update(): void
     {
         try {
             if (cannot('edit', PRIV_CUSTOMERS)) {
@@ -287,7 +287,7 @@ class Customers extends EA_Controller
     /**
      * Remove a customer.
      */
-    public function destroy()
+    public function destroy(): void
     {
         try {
             if (cannot('delete', PRIV_CUSTOMERS)) {
diff --git a/application/controllers/General_settings.php b/application/controllers/General_settings.php
index 06354fc9..08c1f436 100644
--- a/application/controllers/General_settings.php
+++ b/application/controllers/General_settings.php
@@ -36,7 +36,7 @@ class General_settings extends EA_Controller
     /**
      * Render the settings page.
      */
-    public function index()
+    public function index(): void
     {
         session(['dest_url' => site_url('general_settings')]);
 
@@ -81,7 +81,7 @@ class General_settings extends EA_Controller
     /**
      * Save general settings.
      */
-    public function save()
+    public function save(): void
     {
         try {
             if (cannot('edit', PRIV_SYSTEM_SETTINGS)) {
diff --git a/application/controllers/Google_analytics_settings.php b/application/controllers/Google_analytics_settings.php
index 18c49e6a..ec48ce72 100644
--- a/application/controllers/Google_analytics_settings.php
+++ b/application/controllers/Google_analytics_settings.php
@@ -35,7 +35,7 @@ class Google_analytics_settings extends EA_Controller
     /**
      * Render the settings page.
      */
-    public function index()
+    public function index(): void
     {
         session(['dest_url' => site_url('google_analytics_settings')]);
 
@@ -71,7 +71,7 @@ class Google_analytics_settings extends EA_Controller
     /**
      * Save general settings.
      */
-    public function save()
+    public function save(): void
     {
         try {
             if (cannot('edit', PRIV_SYSTEM_SETTINGS)) {
diff --git a/application/controllers/Installation.php b/application/controllers/Installation.php
index 64a1b513..3cb571c9 100644
--- a/application/controllers/Installation.php
+++ b/application/controllers/Installation.php
@@ -39,7 +39,7 @@ class Installation extends EA_Controller
     /**
      * Display the installation page.
      */
-    public function index()
+    public function index(): void
     {
         if (is_app_installed()) {
             redirect();
diff --git a/application/controllers/Integrations.php b/application/controllers/Integrations.php
index ccc76a15..1f76480b 100644
--- a/application/controllers/Integrations.php
+++ b/application/controllers/Integrations.php
@@ -44,7 +44,7 @@ class Integrations extends EA_Controller
     /**
      * Render the settings page.
      */
-    public function index()
+    public function index(): void
     {
         session(['dest_url' => site_url('about')]);
 
diff --git a/application/controllers/Legal_settings.php b/application/controllers/Legal_settings.php
index f327b09c..aa16cda0 100644
--- a/application/controllers/Legal_settings.php
+++ b/application/controllers/Legal_settings.php
@@ -35,7 +35,7 @@ class Legal_settings extends EA_Controller
     /**
      * Render the settings page.
      */
-    public function index()
+    public function index(): void
     {
         session(['dest_url' => site_url('legal_settings')]);
 
@@ -71,7 +71,7 @@ class Legal_settings extends EA_Controller
     /**
      * Save legal settings.
      */
-    public function save()
+    public function save(): void
     {
         try {
             if (cannot('edit', PRIV_SYSTEM_SETTINGS)) {
diff --git a/application/controllers/Localization.php b/application/controllers/Localization.php
index 1cb6aa99..e7d605ef 100644
--- a/application/controllers/Localization.php
+++ b/application/controllers/Localization.php
@@ -27,7 +27,7 @@ class Localization extends EA_Controller
      *
      * Notice: This method used to be in the Backend_api.php.
      */
-    public function change_language()
+    public function change_language(): void
     {
         try {
             // Check if language exists in the available languages.
diff --git a/application/controllers/Login.php b/application/controllers/Login.php
index 735d2d23..65884558 100644
--- a/application/controllers/Login.php
+++ b/application/controllers/Login.php
@@ -38,7 +38,7 @@ class Login extends EA_Controller
     /**
      * Render the login page.
      */
-    public function index()
+    public function index(): void
     {
         if (session('user_id')) {
             redirect('calendar');
@@ -58,7 +58,7 @@ class Login extends EA_Controller
     /**
      * Validate the provided credentials and start a new session if the validation was successful.
      */
-    public function validate()
+    public function validate(): void
     {
         try {
             $username = request('username');
diff --git a/application/controllers/Logout.php b/application/controllers/Logout.php
index bab759b1..144099ed 100644
--- a/application/controllers/Logout.php
+++ b/application/controllers/Logout.php
@@ -23,7 +23,7 @@ class Logout extends EA_Controller
     /**
      * Render the logout page.
      */
-    public function index()
+    public function index(): void
     {
         $this->session->sess_destroy();
 
diff --git a/application/controllers/Matomo_analytics_settings.php b/application/controllers/Matomo_analytics_settings.php
index f77cf558..b643db60 100644
--- a/application/controllers/Matomo_analytics_settings.php
+++ b/application/controllers/Matomo_analytics_settings.php
@@ -35,7 +35,7 @@ class Matomo_analytics_settings extends EA_Controller
     /**
      * Render the settings page.
      */
-    public function index()
+    public function index(): void
     {
         session(['dest_url' => site_url('matomo_analytics_settings')]);
 
@@ -71,7 +71,7 @@ class Matomo_analytics_settings extends EA_Controller
     /**
      * Save general settings.
      */
-    public function save()
+    public function save(): void
     {
         try {
             if (cannot('edit', PRIV_SYSTEM_SETTINGS)) {
diff --git a/application/controllers/Privacy.php b/application/controllers/Privacy.php
index 2e1b1be1..08d14b5f 100644
--- a/application/controllers/Privacy.php
+++ b/application/controllers/Privacy.php
@@ -35,7 +35,7 @@ class Privacy extends EA_Controller
     /**
      * Remove all customer data (including appointments) from the system.
      */
-    public function delete_personal_information()
+    public function delete_personal_information(): void
     {
         try {
             $display_delete_personal_information = setting('display_delete_personal_information');
diff --git a/application/controllers/Providers.php b/application/controllers/Providers.php
index ee3ca765..67552880 100644
--- a/application/controllers/Providers.php
+++ b/application/controllers/Providers.php
@@ -42,7 +42,7 @@ class Providers extends EA_Controller
      * On this page admin users will be able to manage providers, which are eventually selected by customers during the
      * booking process.
      */
-    public function index()
+    public function index(): void
     {
         session(['dest_url' => site_url('providers')]);
 
@@ -95,7 +95,7 @@ class Providers extends EA_Controller
     /**
      * Filter providers by the provided keyword.
      */
-    public function search()
+    public function search(): void
     {
         try {
             if (cannot('view', PRIV_USERS)) {
@@ -121,7 +121,7 @@ class Providers extends EA_Controller
     /**
      * Store a new provider.
      */
-    public function store()
+    public function store(): void
     {
         try {
             if (cannot('add', PRIV_USERS)) {
@@ -180,7 +180,7 @@ class Providers extends EA_Controller
     /**
      * Find a provider.
      */
-    public function find()
+    public function find(): void
     {
         try {
             if (cannot('view', PRIV_USERS)) {
@@ -200,7 +200,7 @@ class Providers extends EA_Controller
     /**
      * Update a provider.
      */
-    public function update()
+    public function update(): void
     {
         try {
             if (cannot('edit', PRIV_USERS)) {
@@ -260,7 +260,7 @@ class Providers extends EA_Controller
     /**
      * Remove a provider.
      */
-    public function destroy()
+    public function destroy(): void
     {
         try {
             if (cannot('delete', PRIV_USERS)) {
diff --git a/application/controllers/Recovery.php b/application/controllers/Recovery.php
index ce6f156c..dc257315 100644
--- a/application/controllers/Recovery.php
+++ b/application/controllers/Recovery.php
@@ -34,7 +34,7 @@ class Recovery extends EA_Controller
     /**
      * Display the password recovery page.
      */
-    public function index()
+    public function index(): void
     {
         $company_name = setting('company_name');
 
diff --git a/application/controllers/Secretaries.php b/application/controllers/Secretaries.php
index 196a8450..65ffc1ae 100644
--- a/application/controllers/Secretaries.php
+++ b/application/controllers/Secretaries.php
@@ -42,7 +42,7 @@ class Secretaries extends EA_Controller
      * On this page secretary users will be able to manage secretaries, which are eventually selected by customers during the
      * booking process.
      */
-    public function index()
+    public function index(): void
     {
         session(['dest_url' => site_url('secretaries')]);
 
@@ -91,7 +91,7 @@ class Secretaries extends EA_Controller
     /**
      * Filter secretaries by the provided keyword.
      */
-    public function search()
+    public function search(): void
     {
         try {
             if (cannot('view', PRIV_USERS)) {
@@ -117,7 +117,7 @@ class Secretaries extends EA_Controller
     /**
      * Store a new secretary.
      */
-    public function store()
+    public function store(): void
     {
         try {
             if (cannot('add', PRIV_USERS)) {
@@ -174,7 +174,7 @@ class Secretaries extends EA_Controller
     /**
      * Find a secretary.
      */
-    public function find()
+    public function find(): void
     {
         try {
             if (cannot('view', PRIV_USERS)) {
@@ -194,7 +194,7 @@ class Secretaries extends EA_Controller
     /**
      * Update a secretary.
      */
-    public function update()
+    public function update(): void
     {
         try {
             if (cannot('edit', PRIV_USERS)) {
@@ -252,7 +252,7 @@ class Secretaries extends EA_Controller
     /**
      * Remove a secretary.
      */
-    public function destroy()
+    public function destroy(): void
     {
         try {
             if (cannot('delete', PRIV_USERS)) {
diff --git a/application/controllers/Service_categories.php b/application/controllers/Service_categories.php
index 32e96c6a..ad53e542 100644
--- a/application/controllers/Service_categories.php
+++ b/application/controllers/Service_categories.php
@@ -41,7 +41,7 @@ class Service_categories extends EA_Controller
      * On this page admin users will be able to manage service-categories, which are eventually selected by customers during the
      * booking process.
      */
-    public function index()
+    public function index(): void
     {
         session(['dest_url' => site_url('service_categories')]);
 
@@ -78,7 +78,7 @@ class Service_categories extends EA_Controller
     /**
      * Filter service-categories by the provided keyword.
      */
-    public function search()
+    public function search(): void
     {
         try {
             if (cannot('view', PRIV_SERVICES)) {
@@ -104,7 +104,7 @@ class Service_categories extends EA_Controller
     /**
      * Store a new service-category.
      */
-    public function store()
+    public function store(): void
     {
         try {
             if (cannot('add', PRIV_SERVICES)) {
@@ -133,7 +133,7 @@ class Service_categories extends EA_Controller
     /**
      * Find a service-category.
      */
-    public function find()
+    public function find(): void
     {
         try {
             if (cannot('view', PRIV_SERVICES)) {
@@ -153,7 +153,7 @@ class Service_categories extends EA_Controller
     /**
      * Update a service-category.
      */
-    public function update()
+    public function update(): void
     {
         try {
             if (cannot('edit', PRIV_SERVICES)) {
@@ -182,7 +182,7 @@ class Service_categories extends EA_Controller
     /**
      * Remove a service-category.
      */
-    public function destroy()
+    public function destroy(): void
     {
         try {
             if (cannot('delete', PRIV_SERVICES)) {
diff --git a/application/controllers/Services.php b/application/controllers/Services.php
index 12c65c05..aff2aa96 100644
--- a/application/controllers/Services.php
+++ b/application/controllers/Services.php
@@ -41,7 +41,7 @@ class Services extends EA_Controller
      * On this page admin users will be able to manage services, which are eventually selected by customers during the
      * booking process.
      */
-    public function index()
+    public function index(): void
     {
         session(['dest_url' => site_url('services')]);
 
@@ -79,7 +79,7 @@ class Services extends EA_Controller
     /**
      * Filter services by the provided keyword.
      */
-    public function search()
+    public function search(): void
     {
         try {
             if (cannot('view', PRIV_SERVICES)) {
@@ -105,7 +105,7 @@ class Services extends EA_Controller
     /**
      * Store a new service.
      */
-    public function store()
+    public function store(): void
     {
         try {
             if (cannot('add', PRIV_SERVICES)) {
@@ -150,7 +150,7 @@ class Services extends EA_Controller
     /**
      * Find a service.
      */
-    public function find()
+    public function find(): void
     {
         try {
             if (cannot('delete', PRIV_SERVICES)) {
@@ -170,7 +170,7 @@ class Services extends EA_Controller
     /**
      * Update a service.
      */
-    public function update()
+    public function update(): void
     {
         try {
             if (cannot('edit', PRIV_SERVICES)) {
@@ -216,7 +216,7 @@ class Services extends EA_Controller
     /**
      * Remove a service.
      */
-    public function destroy()
+    public function destroy(): void
     {
         try {
             if (cannot('delete', PRIV_SERVICES)) {
diff --git a/application/controllers/Unavailabilities.php b/application/controllers/Unavailabilities.php
index 368ae794..ad19fd67 100644
--- a/application/controllers/Unavailabilities.php
+++ b/application/controllers/Unavailabilities.php
@@ -38,7 +38,7 @@ class Unavailabilities extends EA_Controller
     /**
      * Filter unavailabilities by the provided keyword.
      */
-    public function search()
+    public function search(): void
     {
         try {
             if (cannot('view', PRIV_APPOINTMENTS)) {
@@ -64,7 +64,7 @@ class Unavailabilities extends EA_Controller
     /**
      * Store a new unavailability.
      */
-    public function store()
+    public function store(): void
     {
         try {
             if (cannot('add', PRIV_APPOINTMENTS)) {
@@ -109,7 +109,7 @@ class Unavailabilities extends EA_Controller
     /**
      * Find an unavailability.
      */
-    public function find()
+    public function find(): void
     {
         try {
             if (cannot('view', PRIV_APPOINTMENTS)) {
@@ -129,7 +129,7 @@ class Unavailabilities extends EA_Controller
     /**
      * Update a unavailability.
      */
-    public function update()
+    public function update(): void
     {
         try {
             if (cannot('edit', PRIV_APPOINTMENTS)) {
@@ -160,7 +160,7 @@ class Unavailabilities extends EA_Controller
     /**
      * Remove a unavailability.
      */
-    public function destroy()
+    public function destroy(): void
     {
         try {
             if (cannot('delete', PRIV_APPOINTMENTS)) {
diff --git a/application/controllers/Update.php b/application/controllers/Update.php
index f083314a..a4eae4ab 100644
--- a/application/controllers/Update.php
+++ b/application/controllers/Update.php
@@ -45,7 +45,7 @@ class Update extends EA_Controller
      * This method can be used either by loading the page in the browser or by an ajax request. But it will answer with
      * JSON encoded data.
      */
-    public function index()
+    public function index(): void
     {
         try {
             $user_id = session('user_id');
diff --git a/application/controllers/User.php b/application/controllers/User.php
index 10af0024..a71df12c 100644
--- a/application/controllers/User.php
+++ b/application/controllers/User.php
@@ -34,7 +34,7 @@ class User extends EA_Controller
     /**
      * Redirect to the login page.
      */
-    public function index()
+    public function index(): void
     {
         redirect('login');
     }
diff --git a/application/controllers/Webhooks.php b/application/controllers/Webhooks.php
index 57b3045c..3314d7f4 100644
--- a/application/controllers/Webhooks.php
+++ b/application/controllers/Webhooks.php
@@ -40,7 +40,7 @@ class Webhooks extends EA_Controller
      * On this page admin users will be able to manage webhooks, which are eventually selected by customers during the
      * booking process.
      */
-    public function index()
+    public function index(): void
     {
         session(['dest_url' => site_url('webhooks')]);
 
@@ -97,7 +97,7 @@ class Webhooks extends EA_Controller
     /**
      * Filter webhooks by the provided keyword.
      */
-    public function search()
+    public function search(): void
     {
         try {
             if (cannot('view', PRIV_WEBHOOKS)) {
@@ -123,7 +123,7 @@ class Webhooks extends EA_Controller
     /**
      * Store a new webhook.
      */
-    public function store()
+    public function store(): void
     {
         try {
             if (cannot('add', PRIV_WEBHOOKS)) {
@@ -155,7 +155,7 @@ class Webhooks extends EA_Controller
     /**
      * Update a webhook.
      */
-    public function update()
+    public function update(): void
     {
         try {
             if (cannot('edit', PRIV_WEBHOOKS)) {
@@ -188,7 +188,7 @@ class Webhooks extends EA_Controller
     /**
      * Remove a webhook.
      */
-    public function destroy()
+    public function destroy(): void
     {
         try {
             if (cannot('delete', PRIV_WEBHOOKS)) {
@@ -210,7 +210,7 @@ class Webhooks extends EA_Controller
     /**
      * Find a webhook.
      */
-    public function find()
+    public function find(): void
     {
         try {
             if (cannot('view', PRIV_WEBHOOKS)) {
diff --git a/application/controllers/api/v1/Settings_api_v1.php b/application/controllers/api/v1/Settings_api_v1.php
index e32f237c..ee8cbb13 100644
--- a/application/controllers/api/v1/Settings_api_v1.php
+++ b/application/controllers/api/v1/Settings_api_v1.php
@@ -35,7 +35,7 @@ class Settings_api_v1 extends EA_Controller
     /**
      * Get a setting collection.
      */
-    public function index()
+    public function index(): void
     {
         try {
             $keyword = $this->api->request_keyword();
diff --git a/application/models/Admins_model.php b/application/models/Admins_model.php
index 8fe7d650..ac5fc5d8 100644
--- a/application/models/Admins_model.php
+++ b/application/models/Admins_model.php
@@ -76,7 +76,7 @@ class Admins_model extends EA_Model
      *
      * @throws InvalidArgumentException
      */
-    public function validate(array $admin)
+    public function validate(array $admin): void
     {
         // If an admin ID is provided then check whether the record really exists in the database.
         if (!empty($admin['id'])) {
@@ -277,7 +277,7 @@ class Admins_model extends EA_Model
      *
      * @throws InvalidArgumentException
      */
-    protected function save_settings(int $admin_id, array $settings)
+    protected function save_settings(int $admin_id, array $settings): void
     {
         if (empty($settings)) {
             throw new InvalidArgumentException('The settings argument cannot be empty.');
@@ -302,7 +302,7 @@ class Admins_model extends EA_Model
      * @param string $name Setting name.
      * @param mixed|null $value Setting value.
      */
-    public function set_setting(int $admin_id, string $name, mixed $value = null)
+    public function set_setting(int $admin_id, string $name, mixed $value = null): void
     {
         if (!$this->db->update('user_settings', [$name => $value], ['id_users' => $admin_id])) {
             throw new RuntimeException('Could not set the new admin setting value: ' . $name);
@@ -534,7 +534,7 @@ class Admins_model extends EA_Model
      *
      * @param array $admin Admin data.
      */
-    public function api_encode(array &$admin)
+    public function api_encode(array &$admin): void
     {
         $encoded_resource = [
             'id' => array_key_exists('id', $admin) ? (int) $admin['id'] : null,
@@ -565,7 +565,7 @@ class Admins_model extends EA_Model
      * @param array $admin API resource.
      * @param array|null $base Base admin data to be overwritten with the provided values (useful for updates).
      */
-    public function api_decode(array &$admin, array $base = null)
+    public function api_decode(array &$admin, array $base = null): void
     {
         $decoded_resource = $base ?? [];
 
diff --git a/application/models/Blocked_periods_model.php b/application/models/Blocked_periods_model.php
index e42c5846..f8cd04ef 100644
--- a/application/models/Blocked_periods_model.php
+++ b/application/models/Blocked_periods_model.php
@@ -46,6 +46,7 @@ class Blocked_periods_model extends EA_Model
      * @return int Returns the blocked-period ID.
      *
      * @throws InvalidArgumentException
+     * @throws Exception
      */
     public function save(array $blocked_period): int
     {
@@ -64,8 +65,9 @@ class Blocked_periods_model extends EA_Model
      * @param array $blocked_period Associative array with the blocked-period data.
      *
      * @throws InvalidArgumentException
+     * @throws Exception
      */
-    public function validate(array $blocked_period)
+    public function validate(array $blocked_period): void
     {
         // If a blocked-period ID is provided then check whether the record really exists in the database.
         if (!empty($blocked_period['id'])) {
@@ -301,7 +303,7 @@ class Blocked_periods_model extends EA_Model
      *
      * @param array $blocked_period Blocked period data.
      */
-    public function api_encode(array &$blocked_period)
+    public function api_encode(array &$blocked_period): void
     {
         $encoded_resource = [
             'id' => array_key_exists('id', $blocked_period) ? (int) $blocked_period['id'] : null,
@@ -320,7 +322,7 @@ class Blocked_periods_model extends EA_Model
      * @param array $blocked_period API resource.
      * @param array|null $base Base blocked-period data to be overwritten with the provided values (useful for updates).
      */
-    public function api_decode(array &$blocked_period, array $base = null)
+    public function api_decode(array &$blocked_period, array $base = null): void
     {
         $decoded_resource = $base ?: [];
 
diff --git a/application/models/Consents_model.php b/application/models/Consents_model.php
index a58b247b..c7b8fc91 100644
--- a/application/models/Consents_model.php
+++ b/application/models/Consents_model.php
@@ -54,7 +54,7 @@ class Consents_model extends EA_Model
      *
      * @throws InvalidArgumentException
      */
-    public function validate(array $consent)
+    public function validate(array $consent): void
     {
         if (empty($consent['ip']) || empty($consent['type'])) {
             throw new InvalidArgumentException('Not all required fields are provided: ' . print_r($consent, true));
diff --git a/application/models/Customers_model.php b/application/models/Customers_model.php
index d97ac0f7..c698fd41 100644
--- a/application/models/Customers_model.php
+++ b/application/models/Customers_model.php
@@ -82,7 +82,7 @@ class Customers_model extends EA_Model
      *
      * @throws InvalidArgumentException
      */
-    public function validate(array $customer)
+    public function validate(array $customer): void
     {
         // If a customer ID is provided then check whether the record really exists in the database.
         if (!empty($customer['id'])) {
@@ -443,7 +443,7 @@ class Customers_model extends EA_Model
      *
      * @param array $customer Customer data.
      */
-    public function api_encode(array &$customer)
+    public function api_encode(array &$customer): void
     {
         $encoded_resource = [
             'id' => array_key_exists('id', $customer) ? (int) $customer['id'] : null,
@@ -472,7 +472,7 @@ class Customers_model extends EA_Model
      * @param array $customer API resource.
      * @param array|null $base Base customer data to be overwritten with the provided values (useful for updates).
      */
-    public function api_decode(array &$customer, array $base = null)
+    public function api_decode(array &$customer, array $base = null): void
     {
         $decoded_resource = $base ?: [];
 
diff --git a/application/models/Roles_model.php b/application/models/Roles_model.php
index 7f71d6f8..7f7fefd8 100644
--- a/application/models/Roles_model.php
+++ b/application/models/Roles_model.php
@@ -61,7 +61,7 @@ class Roles_model extends EA_Model
      *
      * @throws InvalidArgumentException
      */
-    public function validate(array $role)
+    public function validate(array $role): void
     {
         // If a role ID is provided then check whether the record really exists in the database.
         if (!empty($role['id'])) {
diff --git a/application/models/Secretaries_model.php b/application/models/Secretaries_model.php
index d76428a9..904e72f3 100644
--- a/application/models/Secretaries_model.php
+++ b/application/models/Secretaries_model.php
@@ -76,7 +76,7 @@ class Secretaries_model extends EA_Model
      *
      * @throws InvalidArgumentException
      */
-    public function validate(array $secretary)
+    public function validate(array $secretary): void
     {
         // If a secretary ID is provided then check whether the record really exists in the database.
         if (!empty($secretary['id'])) {
@@ -306,7 +306,7 @@ class Secretaries_model extends EA_Model
      *
      * @throws InvalidArgumentException
      */
-    protected function save_settings(int $secretary_id, array $settings)
+    protected function save_settings(int $secretary_id, array $settings): void
     {
         if (empty($settings)) {
             throw new InvalidArgumentException('The settings argument cannot be empty.');
@@ -331,7 +331,7 @@ class Secretaries_model extends EA_Model
      * @param string $name Setting name.
      * @param mixed|null $value Setting value.
      */
-    public function set_setting(int $secretary_id, string $name, mixed $value = null)
+    public function set_setting(int $secretary_id, string $name, mixed $value = null): void
     {
         if (!$this->db->update('user_settings', [$name => $value], ['id_users' => $secretary_id])) {
             throw new RuntimeException('Could not set the new secretary setting value: ' . $name);
@@ -387,7 +387,7 @@ class Secretaries_model extends EA_Model
      * @param int $secretary_id Secretary ID.
      * @param array $provider_ids Provider IDs.
      */
-    protected function save_provider_ids(int $secretary_id, array $provider_ids)
+    protected function save_provider_ids(int $secretary_id, array $provider_ids): void
     {
         // Re-insert the secretary-provider connections.
         $this->db->delete('secretaries_providers', ['id_users_secretary' => $secretary_id]);
@@ -554,7 +554,7 @@ class Secretaries_model extends EA_Model
      *
      * @throws InvalidArgumentException
      */
-    public function load(array &$secretary, array $resources)
+    public function load(array &$secretary, array $resources): void
     {
         if (empty($secretary) || empty($resources)) {
             return;
@@ -581,7 +581,7 @@ class Secretaries_model extends EA_Model
      *
      * @param array $secretary Secretary data.
      */
-    public function api_encode(array &$secretary)
+    public function api_encode(array &$secretary): void
     {
         $encoded_resource = [
             'id' => array_key_exists('id', $secretary) ? (int) $secretary['id'] : null,
@@ -613,7 +613,7 @@ class Secretaries_model extends EA_Model
      * @param array $secretary API resource.
      * @param array|null $base Base secretary data to be overwritten with the provided values (useful for updates).
      */
-    public function api_decode(array &$secretary, array $base = null)
+    public function api_decode(array &$secretary, array $base = null): void
     {
         $decoded_resource = $base ?: [];
 
diff --git a/application/models/Service_categories_model.php b/application/models/Service_categories_model.php
index 88d624d2..c0dda2e4 100644
--- a/application/models/Service_categories_model.php
+++ b/application/models/Service_categories_model.php
@@ -63,7 +63,7 @@ class Service_categories_model extends EA_Model
      *
      * @throws InvalidArgumentException
      */
-    public function validate(array $service_category)
+    public function validate(array $service_category): void
     {
         // If a service-category ID is provided then check whether the record really exists in the database.
         if (!empty($service_category['id'])) {
@@ -297,7 +297,7 @@ class Service_categories_model extends EA_Model
      *
      * @param array $service_category Category data.
      */
-    public function api_encode(array &$service_category)
+    public function api_encode(array &$service_category): void
     {
         $encoded_resource = [
             'id' => array_key_exists('id', $service_category) ? (int) $service_category['id'] : null,
@@ -316,7 +316,7 @@ class Service_categories_model extends EA_Model
      * @param array $service_category API resource.
      * @param array|null $base Base service-category data to be overwritten with the provided values (useful for updates).
      */
-    public function api_decode(array &$service_category, array $base = null)
+    public function api_decode(array &$service_category, array $base = null): void
     {
         $decoded_resource = $base ?: [];
 
diff --git a/application/models/Services_model.php b/application/models/Services_model.php
index 388610ea..f117f83b 100644
--- a/application/models/Services_model.php
+++ b/application/models/Services_model.php
@@ -76,7 +76,7 @@ class Services_model extends EA_Model
      *
      * @throws InvalidArgumentException
      */
-    public function validate(array $service)
+    public function validate(array $service): void
     {
         // If a service ID is provided then check whether the record really exists in the database.
         if (!empty($service['id'])) {
@@ -380,7 +380,7 @@ class Services_model extends EA_Model
      *
      * @throws InvalidArgumentException
      */
-    public function load(array &$service, array $resources)
+    public function load(array &$service, array $resources): void
     {
         if (empty($service) || empty($resources)) {
             return;
@@ -405,7 +405,7 @@ class Services_model extends EA_Model
      *
      * @param array $service Service data.
      */
-    public function api_encode(array &$service)
+    public function api_encode(array &$service): void
     {
         $encoded_resource = [
             'id' => array_key_exists('id', $service) ? (int) $service['id'] : null,
@@ -430,7 +430,7 @@ class Services_model extends EA_Model
      * @param array $service API resource.
      * @param array|null $base Base service data to be overwritten with the provided values (useful for updates).
      */
-    public function api_decode(array &$service, array $base = null)
+    public function api_decode(array &$service, array $base = null): void
     {
         $decoded_resource = $base ?: [];
 
diff --git a/application/models/Settings_model.php b/application/models/Settings_model.php
index a3fb7416..93dc8030 100644
--- a/application/models/Settings_model.php
+++ b/application/models/Settings_model.php
@@ -62,7 +62,7 @@ class Settings_model extends EA_Model
      *
      * @throws InvalidArgumentException
      */
-    public function validate(array $setting)
+    public function validate(array $setting): void
     {
         // If a setting ID is provided then check whether the record really exists in the database.
         if (!empty($setting['id'])) {
@@ -288,7 +288,7 @@ class Settings_model extends EA_Model
      *
      * @param array $setting Setting data.
      */
-    public function api_encode(array &$setting)
+    public function api_encode(array &$setting): void
     {
         $encoded_resource = [
             'name' => $setting['name'],
@@ -304,7 +304,7 @@ class Settings_model extends EA_Model
      * @param array $setting API resource.
      * @param array|null $base Base setting data to be overwritten with the provided values (useful for updates).
      */
-    public function api_decode(array &$setting, array $base = null)
+    public function api_decode(array &$setting, array $base = null): void
     {
         $decoded_resource = $base ?: [];
 
diff --git a/application/models/Unavailabilities_model.php b/application/models/Unavailabilities_model.php
index d2711e52..86ad4253 100644
--- a/application/models/Unavailabilities_model.php
+++ b/application/models/Unavailabilities_model.php
@@ -73,7 +73,7 @@ class Unavailabilities_model extends EA_Model
      *
      * @throws InvalidArgumentException
      */
-    public function validate(array $unavailability)
+    public function validate(array $unavailability): void
     {
         // If an unavailability ID is provided then check whether the record really exists in the database.
         if (!empty($unavailability['id'])) {
@@ -349,7 +349,7 @@ class Unavailabilities_model extends EA_Model
      *
      * @throws InvalidArgumentException
      */
-    public function load(array &$unavailability, array $resources)
+    public function load(array &$unavailability, array $resources): void
     {
         if (empty($unavailability) || empty($resources)) {
             return;
@@ -374,7 +374,7 @@ class Unavailabilities_model extends EA_Model
      *
      * @param array $unavailability Unavailability data.
      */
-    public function api_encode(array &$unavailability)
+    public function api_encode(array &$unavailability): void
     {
         $encoded_resource = [
             'id' => array_key_exists('id', $unavailability) ? (int) $unavailability['id'] : null,
@@ -399,7 +399,7 @@ class Unavailabilities_model extends EA_Model
      * @param array $unavailability API resource.
      * @param array|null $base Base unavailability data to be overwritten with the provided values (useful for updates).
      */
-    public function api_decode(array &$unavailability, array $base = null)
+    public function api_decode(array &$unavailability, array $base = null): void
     {
         $decoded_request = $base ?: [];
 
diff --git a/application/models/Users_model.php b/application/models/Users_model.php
index 3bc01245..1b071234 100644
--- a/application/models/Users_model.php
+++ b/application/models/Users_model.php
@@ -76,7 +76,7 @@ class Users_model extends EA_Model
      *
      * @throws InvalidArgumentException
      */
-    public function validate(array $user)
+    public function validate(array $user): void
     {
         // If a user ID is provided then check whether the record really exists in the database.
         if (!empty($user['id'])) {
@@ -138,7 +138,7 @@ class Users_model extends EA_Model
      *
      * @throws InvalidArgumentException
      */
-    protected function save_settings(int $user_id, array $settings)
+    protected function save_settings(int $user_id, array $settings): void
     {
         if (empty($settings)) {
             throw new InvalidArgumentException('The settings argument cannot be empty.');
@@ -163,7 +163,7 @@ class Users_model extends EA_Model
      * @param string $name Setting name.
      * @param string $value Setting value.
      */
-    public function set_setting(int $user_id, string $name, string $value)
+    public function set_setting(int $user_id, string $name, string $value): void
     {
         if (!$this->db->update('user_settings', [$name => $value], ['id_users' => $user_id])) {
             throw new RuntimeException('Could not set the new user setting value: ' . $name);
diff --git a/application/models/Webhooks_model.php b/application/models/Webhooks_model.php
index 5ccc936a..e7eb1680 100644
--- a/application/models/Webhooks_model.php
+++ b/application/models/Webhooks_model.php
@@ -70,7 +70,7 @@ class Webhooks_model extends EA_Model
      *
      * @throws InvalidArgumentException
      */
-    public function validate(array $webhook)
+    public function validate(array $webhook): void
     {
         if (empty($webhook['name']) || empty($webhook['url']) || empty($webhook['actions'])) {
             throw new InvalidArgumentException('Not all required fields are provided: ' . print_r($webhook, true));
@@ -304,7 +304,7 @@ class Webhooks_model extends EA_Model
      * @param array $webhook API resource.
      * @param array|null $base Base webhook data to be overwritten with the provided values (useful for updates).
      */
-    public function api_decode(array &$webhook, array $base = null)
+    public function api_decode(array &$webhook, array $base = null): void
     {
         $decoded_resource = $base ?: [];