- floor: documentation ( source)
- date: documentation ( source)
- strtotime: documentation ( source)
<?php
$bithdayDate = date("Y-m-d",strtotime("21/07/1986"));
$bithdayDate = strtotime($bithdayDate);
$tdate = strtotime(date("Y-m-d"));
echo floor(($tdate - $bithdayDate)/31556926);