关于这个月剩多少天，php的如何计算

$today = time();
	$todaydate = date('m,Y,d',$today);
	$todaydate = explode(',',$todaydate);
	echo $residue = cal_days_in_month(CAL_GREGORIAN, $todaydate[0],$todaydate[1]) - $todaydate[2];