public function down()
    {
        header('content-type:application:json;charset=utf8');
        header('Access-Control-Allow-Origin:https://coucouchina.com');
        header('Access-Control-Allow-Methods:POST');
        header('Access-Control-Allow-Headers:x-requested-with,content-type');
        header("Content-type:text/html;charset=utf-8"
 
);
// $file_name="cookie.jpg";
        $file_name = "apple-app-site-association";
//用以解決中文不能顯示出來的問題
        $file_name = iconv("utf-8", "gb2312", $file_name);
        $file_sub_path = $_SERVER['DOCUMENT_ROOT'] . "/";
        $file_path = $file_sub_path . $file_name;
//首先要判斷給定的檔案存在與否
        if (!file_exists($file_path
 
)) {
            echo "沒有該檔案檔案";
            return;
        }
        $fp = fopen($file_path, "r");
        $file_size = filesize($file_path);
//下載檔案需要用到的頭
        Header("Content-type: application/octet-stream");
        Header("Accept-Ranges: bytes");
        Header("Accept-Length:" . $file_size);
        Header("Content-Disposition: attachment; filename="
 
 . $file_name);
        $buffer = 1024;
        $file_count = 0;
//向瀏覽器返回資料
        while (!feof($fp) && $file_count < $file_size) {
            $file_con = fread($fp, $buffer);
            $file_count += $buffer;
            echo $file_con;
        }
        fclose($fp);
    }