- var_dump: documentation ( source)
- date_default_timezone_set: documentation ( source)
<?php
date_default_timezone_set('Europe/Berlin');
$birthday = new DateTime('1990-09-23 22:09');
$today = new DateTime('2013-01-24 21:00');
$today->add(new DateInterval('PT1H'));
$age = $birthday->diff($today, true);
var_dump($age->h);