ICal\ICal::isValidTimeZoneId PHP Method

    function isValidTimeZoneId($timezone)
    {
        $valid = array();
        $tza = timezone_abbreviations_list();
        foreach ($tza as $zone) {
            foreach ($zone as $item) {
                $valid[$item['timezone_id']] = true;
            }
        }
        unset($valid['']);
        return isset($valid[$timezone]);
    }