- mktime: documentation ( source)
- date: documentation ( source)
- strtotime: documentation ( source)
<?php
$today_day = 28;
$day = date("d-m-Y", mktime(0,0,0,date("m"), $today_day,date("Y")));
echo $day ."\n";
$today_day++;
$day = date("d-m-Y", mktime(0,0,0,date("m", strtotime($day)),$today_day,date("Y", strtotime($day))));
echo $day;
?>