// 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!');
}