PHP输入指定日期，返回那一周的开始和结束时间

$day = '20190531';
$time = date('Y-m-d',strtotime($day));
$last_day = date("Y-m-d",strtotime("$time Sunday"));
$first_day = date("Y-m-d",strtotime("$last_day - 6 days"));
$last_day = str_replace('-','',$last_day);
$first_day = str_replace('-','',$first_day);