php做控制下载速度代码 怎么又生带宽，又省cpu 和内存？

<code> <?php/* Source: http://www.apphp.com/index.php?snippet=php-download-file-with-speed-limit */    /* set here a limit of downloading rate (e.g. 10.20 Kb/s) */   // $download_rate = 10.20;   $download_rate = 200;   set_time_limit(0);    $download_file = 'myfile.rar';      $target_file = 'PHP敏捷开发CodeIgniter框架.rar';    if(file_exists($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'); //HTTP Content-type （ 二进制流，不知道下载文件类型）        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!');    }    ?></code>