PHP gets time period code

$start_time = '';
$end_time = '';

switch ($type) {
    case 'thisWeek'://本周
        $start_time =  date("Y-m-d H:i:s",strtotime("-0 week Monday"));
        $end_time =  date("Y-m-d H:i:s",strtotime("-0 week Sunday"));
        break;
    case 'lastWeek'://上周
        $start_time =  date("Y-m-d H:i:s",strtotime("-1 week Monday"));
        $end_time =  date("Y-m-d H:i:s",strtotime("-1 week Sunday"));
        break;
    case 'thisMonth'://本月
        $start_time =  date("Y-m-d H:i:s",strtotime("first day of this month"));
        $end_time =  date('Y-m-d H:i:s',strtotime("last day of this month"));
        break;
    case 'lastMonth'://上个月
        $start_time =  date("Y-m-d H:i:s",strtotime("first day of last month"));
        $end_time =  date('Y-m-d H:i:s',strtotime("last day of last month"));
        break;
    case 'lastMonth2'://上上个月
        $start_time =  date("Y-m-d H:i:s",strtotime("first day of last month -1 month"));
        $end_time =  date('Y-m-d H:i:s',strtotime("last day of last month -1 month"));
        break;
    case 'lastMonth3'://上上上个月
        $start_time =  date("Y-m-d H:i:s",strtotime("first day of last month -2 month"));
        $end_time =  date('Y-m-d H:i:s',strtotime("last day of last month -2 month"));
        break;
    default :
        $start_time = date("Y-m-d H:i:s");
        $end_time = date("Y-m-d H:i:s");
}

PHP