php控制文件下载速度的方法

<?php
 /*
 * set here a limit of downloading rate (e.g. 10.20 kb/s)
 */
 $download_rate = 10.20;
 $download_file = 'download-file.zip'; 
 $target_file = 'target-file.zip';
 if(file_exists($download_file)){
  /* headers */
  header('last-modified:'.gmdate('d, d m y h:i:s').'gmt');
  header('cache-control: private');
  header('content-type: application/octet-stream');
  header('content-length: '.filesize($download_file));
  header('content-disposition: filename='.$target_file);
  /* flush content */
  flush();
  /* open file */
  $fh = @fopen($download_file, 'r');
  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('fatal error: the '.$download_file.' file does not exist!');
 }
?>