All supported timezone identifiers:
<?php
    $ids = timezone_identifiers_list();
    echo "Number of identifiers: ", count($ids), "\n";
    echo implode(", ", array_slice($ids, 0, 5)), "...\n";
    echo implode(", ", array_slice($ids, -5));
?>
Output
Number of identifiers: 419 Africa/Abidjan, Africa/Accra, Africa/Addis_Ababa, Africa/Algiers, Africa/Asmara... Pacific/Tarawa, Pacific/Tongatapu, Pacific/Wake, Pacific/Wallis, UTC