- time: documentation ( source)
- mktime: documentation ( source)
<?php
$target_days=mktime(0,0,0,12,31,2013);//modify the birth day 12/31/2013
$today = time();
$diff_days = ($target_days - $today);
$days = (Int) ($diff_days/86400);
print "days till next birthday: $days days!";