- mktime: documentation ( source)
- date_default_timezone_set: documentation ( source)
- date: documentation ( source)
- strtotime: documentation ( source)
<?php
$time_zones = array(
'America/Chicago',
'GMT',
'Europe/Berlin',
);
foreach($time_zones as $time_zone){
date_default_timezone_set($time_zone);
$t1 = mktime(23, 0, 0);
$t2 = strtotime('today 23:00:00');
echo date('r', $t1) . ' / ' . date('r', $t2) . PHP_EOL;
}