php中如何获取最近六个月每个月的起始时间和结束时间

public function getMonth($m=6) {

        $currentTime = time();

        $cyear = floor(date("Y",$currentTime));

        $cMonth = floor(date("m",$currentTime));

        $months = [];

        for($i=$m-1;$i>=0;$i--){

                $nMonth = $cMonth-$i;
                $cyear = $nMonth == 0 ? ($cyear-1) : $cyear;
                $nMonth = $nMonth <= 0 ? 12+$nMonth : $nMonth;
                $date = $cyear."-".$nMonth."-1";

                $firstday = date('Y-m-01 00:00:00', strtotime($date));

                $lastday = date('Y-m-t 23:59:59', strtotime($date));

                $months[$nMonth] = [
                        'start' => $firstday,
                        'end' => $lastday
                ];

        }
        return $months;
}