php代码实现控制文件下载速度

<?php
 /*
 * set here a limit of downloading rate (e.g. 10.20 Kb/s)
 */
 $download_rate = 10.20;
 $download_file = &#39;download-file.zip&#39;; 
 $target_file = &#39;target-file.zip&#39;;
 if(file_exists($download_file)){
  /* headers */
  header(&#39;Last-Modified:&#39;.gmdate(&#39;D, d M Y H:i:s&#39;).&#39;GMT&#39;);
  header(&#39;Cache-control: private&#39;);
  header(&#39;Content-Type: application/octet-stream&#39;);
  header(&#39;Content-Length: &#39;.filesize($download_file));
  header(&#39;Content-Disposition: filename=&#39;.$target_file);
  /* flush content */
  flush();
  /* open file */
  $fh = @fopen($download_file, &#39;r&#39;);
  while(!feof($fh)){
   /* send only current part of the file to browser */
   print fread($fh, round($download_rate * 1024));
   /* flush the content to the browser */
   flush();
   /* sleep for 1 sec */
   sleep(1);
  }
  /* close file */
  @fclose($fh);
 }else{
  die(&#39;Fatal error: the &#39;.$download_file.&#39; file does not exist!&#39;);
 }
?>