PHP Date-Function to add Calendar-Month to a specified Date and can handle leap years

add_month

<?php
function add_month($startdate,$month) {
  $anyDate = date("Y-m-d",strtotime($startdate . " +1 days"));
  $nextDate = date("Y-m-d",strtotime($anyDate . " +".$month." month"));
  $checkDate = date("Y-m-d",strtotime($nextDate . " -".$month." month"));
	if($anyDate != $checkDate){
	    list($yr,$mn,$da) = split('-',$anyDate);    // separate date
			$mn = $mn + $month;
			$timestamp = mktime(0,0,0,$mn,1,$yr);    
			list($y,$m,$t) = split('-',date('Y-m-t',$timestamp)); 
			$lastDay = date('Y-m-d',mktime(0,0,0,$m,$t,$y)); 
			$return = $lastDay; 
	}
	else $return = date("Y-m-d",strtotime($nextDate . " -1 days")); 
	return $return;
}

//Example use:
echo add_month('2015-01-31',1); // Will retun "2015-02-28"
echo add_month('2015-02-28',1); // Will retun "2015-03-31"

?>