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!'); }?>