php 求某个自然周的第一天 和 求某个自然周的最后一天

 
 //求某个自然周的第一天
    protected function get_firstday($date) {
        $timestamp = strtotime($date);
        $sdate = date('Y-m-d',$timestamp-(date('N',$timestamp)-1)*86400);
        return $sdate;
    }
    //求某个自然周的最后一天
    protected function get_lastday($date){
        $timestamp = strtotime($date);
        $edate = date('Y-m-d',$timestamp + (7-date('N',$timestamp))*86400);
        return $edate;
    }