PHP獲取指定時間的前N個月

function to_last_month($num){
	$today = date("Y-m-d");
	
	$arr = array();
	$m = '-'.$num.' month';
	$old_time = strtotime($m,strtotime($today));
	for($i = 0;$i <= $num-1; ++$i){
		$t = strtotime("+$i month",$old_time);
		$arr[] = date('y年m月',$t);
	}
	//return $str = "'".str_replace( ",","','", implode(',',$arr))."'"; 
	return json_encode($arr);
}