PHP指定月対応日数のコード取得

<?php
/**
*           
* www.jbxue.com
*/
$firstday = mktime(0,0,0,$month,1,$year); //          UNIX       
$days = date('t',$firstday); //           

$days = cal_days_in_month(CAL_GREGORIAN, 4, 2013);//  2003-02     

$days = date('t', strtotime($year . '-' . $month . '-01'));//      

$ndays = date("t"); //return the number of days for this month and this year  
date("j",mktime(0,0,0,$month+1,0,$year));  
?>