PHP function to get the timestamp of the first day of the specified month and the end of the last day

/**
 * Get the timestamp of the first day of the specified month and the end of the last day
 *
 * @param int $y year $m month
 * @return array (start time of this month, end time of this month)
*/
function mFristAndLast($y="",$m=""){
     if($y=="") $y=date("Y");
     if($m=="") $m=date("m");
     $m=sprintf("%02d",intval($m));
     $y=str_pad(intval($y),4,"0",STR_PAD_RIGHT);

     $m>12||$m<1?$m=1:$m=$m;
     $firstday=strtotime($y.$m."01000000");
     $firstdaystr=date("Y-m-01",$firstday);
     $lastday = strtotime(date('Y-m-d 23:59:59', strtotime("$firstdaystr +1 month -1 day")));
     return array("firstday"=>$firstday,"lastday"=>$lastday);
    }

PHP