php protect file with limit download rate

$local_file = 'file.zip';
$download_file = 'name.zip';
// set the download rate limit (=> 20,5 kb/s)
$download_rate = 20.5;

if(file_exists($local_file) && is_file($local_file)){
    header('Cache-control: private');
    header('Content-Type: application/octet-stream');
    header('Content-Length: '.filesize($local_file));
    header('Content-Disposition: filename='.$download_file);
    flush();
    $file = fopen($local_file, "r");
    while(!feof($file)){
        // send the current file part to the browser
        print fread($file, round($download_rate * 1024));
        // flush the content to the browser
        flush();
        // sleep one second
        sleep(1);
    }
    fclose($file);
}else {
    die('Error: The file '.$local_file.' does not exist!');
}