得出当前日期是今年第几周

public function by_week() {
        $day = date("w",time());
        $days = 6-$day;
        $lastime = strtotime("+{$days} day");
        $arr = array();
        for($i=0;$i<20;$i++) {
            $val = strtotime("-{$i} week",$lastime);
            $key = $val.'-'.date("o年W周",$val);
            $val2 = date("o年W周",$val);
            $arr[] = array(
                'key' => strtotime(date("Y-m-d",$val)),
                "week" => date("o年W周",$val)
            );
         }
        return $arr;
    }

  $month = date('m', time()); // 获取本月
   $year = date('Y', time()); // 获取本年
   $firstday = date('w', mktime(0, 0, 0, $month, 1, $year)); //本月1号星期数
   $firstweek = 7 - $firstday; // 第1周天数