php 获取指定月份的开始结束时间戳

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
    );
}