isValidTimezoneId.php

<?php

/**
 * Is Valid Timezone ID
 *
 * @param string $timezone_id Timezone ID to validate.
 *
 * @return bool True if timezone ID is valid; otherwise false.
 */
function is_valid_timezone_id( $timezone_id ){
    $valid = array();
    $tza = timezone_abbreviations_list();

    foreach ( $tza as $zone )
        foreach ( $zone as $item )
            $valid[$item['timezone_id']] = true;

    unset( $valid[''] );

    return !!$valid[$timezone_id];
}