Zastavení, když je proměnná prázdná

if ($type=="download"){

    $bufferSize = 512 * 1024; // 512Kb per second
    ob_end_clean();
    ignore_user_abort(true);
    $output = $url;
    $filename = $url;
        header('Content-Type: application/octet-stream');
        header('Content-Disposition: attachment; filename=' . basename($filename));
        //header('Content-Length: ' . filesize($filename));
        $resource = fopen($filename, 'rb');
        while (!feof($resource)) {
            echo fread($resource, $bufferSize);
            flush();
            //sleep(1);
        }
        fclose($resource);