- mktime: documentation ( source)
- explode: documentation ( source)
- date: documentation ( source)
- strtotime: documentation ( source)
<?php
$birthday = date('Y-m-d', strtotime('2015-02-01'));
$birthDate = explode($birthday, '-');
$age = (date("md", date("U", mktime(0, 0, 0, $birthDate[0], $birthDate[1], $birthDate[2]))) > date("md")
? ((date("Y") - $birthDate[2]) - 1)
: (date("Y") - $birthDate[2]));