Answer by Diego Favero for PHP Timezone List

from @chpx 's answer:

function getAllTimeZones(){$selectOptions = "";    foreach(\DateTimeZone::listIdentifiers() as  $zoneLabel)    {        $currentTimeInZone = new \DateTime("now", new \DateTimeZone($zoneLabel));        $currentTimeDiff = $currentTimeInZone->format('P');        $selectOptions .= "<option value=\"$zoneLabel\">(GMT $currentTimeDiff) $zoneLabel</option>\n";    }return $selectOptions;}