- mktime: documentation ( source)
- explode: documentation ( source)
- date: documentation ( source)
- strtotime: documentation ( source)
<?php
$birthday = date('d-m-Y', strtotime('0000-00-00'));
$birthDate = explode('-', $birthday);
echo $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]));