easyappointments/application/views/partial/timezone_dropdown.php

<select <?= $attributes ?>>
    <?php foreach ($timezones as $continent => $entries): ?>
        <optgroup label="<?= $continent ?>">
            <?php foreach ($entries as $value => $name): ?>
                <option value="<?= $value ?>"><?= $name ?></option>
            <?php endforeach ?>
        </optgroup>
    <?php endforeach ?>
</select>