/**
     * 下载
     * @date: 2021-12-09
     */
    public function downloads()
    {
        set_time_limit(0);
        $res = $_REQUEST;
        $root = $this->root;
        $root = rtrim(rtrim($root, '\\'), '/');
        $path = array_key_exists('path', $res) ? $res['path'] : '';
        $path = trim(trim($path, '\\'), '/');
        $file = $root . $path;
        //检测文件是否存在
        if (file_exists($file)) {
            $name = basename($file);

            $file_size = filesize($file);
            $ranges = $this->getRange($file_size);

            header("Content-type:application/octet-stream");
            header("Accept-Ranges:bytes");
            header('Content-Disposition: attachment; filename=' . $name);
            if ($ranges) {
                $range_start = $ranges['start'];
                $range_end = $ranges['end'];
                header('HTTP/1.1 206 Partial Content');
                header(sprintf('Content-Length: %u', $range_end - $range_start));
                header(sprintf('Content-Range: bytes %s-%s/%s', $range_start, $range_end, $file_size - 1));
            } else {
                header("HTTP/1.1 200 OK");
                header(sprintf('Content-Length: %s', $file_size - 1));
            }

            $fp = fopen($file, 'rb');
            // fp指针跳到断点位置
            $n = empty($ranges['start']) ? 0 : $ranges['start'];
            fseek($fp, $n);

            while (!feof($fp)) {
                echo fread($fp, 1024);
                ob_flush();
                flush();
            }

            fclose($fp);
        } else {
            return '文件不存在或系统错误';
        }
    }
    /** 获取header range信息
     * @param  int   $file_size 文件大小
     * @return Array
     */
    private function getRange($file_size)
    {
        if (isset($_SERVER['HTTP_RANGE']) && !empty($_SERVER['HTTP_RANGE'])) {
            $range = $_SERVER['HTTP_RANGE'];
            $range = preg_replace('/[\s|,].*/', '', $range);
            $range = explode('-', substr($range, 6));
            $arr = [
                'start' => (isset($range[0]) && !empty($range[0])) ? $range[0] : 0,
                'end' => (isset($range[1]) && !empty($range[1]) && $range[1] != 0) ? $range[1] : $file_size - 1,
            ];
            return $arr;
        }
        return null;
    }