function down($url){

        header('Content-Description: File Transfer');

        header('Content-Type: application/octet-stream');

        header('Content-Disposition: attachment; filename='.basename($papers['file_path']));

        header('Content-Transfer-Encoding: binary');

        header('Expires: 0');

        header('Cache-Control: must-revalidate, post-check=0, pre-check=0');

        header('Pragma: public');

        header('Content-Length: ' . $url);

        readfile('location:http://你的域名/'.$url);

    }

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