php 根據周數獲取當周的開始日期與最后日期

   /**
     * 根據第幾周獲取當周的開始日期與最后日期
     * @param int $year 年份 $weeks = get_week($year)
     * @param 如獲取第18周的開始日期$weeks[18][0]
     * @param 如獲取第18周的最后日期$weeks[18][1]
    */
    static  public function get_week($year) {
        $year_start = $year . "-01-01";
        $year_end = $year . "-12-31";
        $startday = strtotime($year_start);
        if (intval(date('N', $startday)) != '1') {
            $startday = strtotime("next monday", strtotime($year_start)); //獲取年第一周的日期
        }
        $year_mondy = date("Y-m-d", $startday); //獲取年第一周的日期

        $endday = strtotime($year_end);
        if (intval(date('W', $endday)) == '7') {
            $endday = strtotime("last sunday", strtotime($year_end));
        }

        $num = intval(date('W', $endday));
        for ($i = 1; $i <= $num; $i++) {
            $j = $i -1;
            $start_date = date("Y-m-d", strtotime("$year_mondy $j week "));

            $end_day = date("Y-m-d", strtotime("$start_date +6 day"));

            $week_array[$i] = array (
                str_replace("-",
                "",
                $start_date
            ), str_replace("-", "", $end_day));
        }
        return $week_array;
    }