PHP限制文件下載的速度

<?php
// 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!');
}
?></pre>