// local file that should be send to the client
$local_file = 'test-file.zip';

// filename that the user gets as default
$download_file = 'your-download-name.zip';

// set the download rate limit (=> 20,5 kb/s)
$download_rate = 20.5;

if(file_exists($local_file) && is_file($local_file)) {

 // send headers
 header('Cache-control: private');
 header('Content-Type: application/octet-stream');
 header('Content-Length: '.filesize($local_file));
 header('Content-Disposition: filename='.$download_file);

 // flush content
 flush();

 // open file stream
 $file = fopen($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);
 }

 // close file stream
 fclose($file);

}

else {
 die('Error: The file '.$local_file.' does not exist!');
}