php下载大文件

<?php
$file = @ fopen($file_dir . $file_name,"r");
$filesize=filesize($file_dir.$file_name);
header('Content-Description:File Transfer');
header("Content-Type:application/octet-stream");
header('Content-Transfer-Encoding:binary');
header("Accept-Ranges: bytes");
header('Expires:0');
header('Cache-Control:must-revalidate');
header('Pragma:public');
header("Content-Length:".$filesize);
header("Content-Disposition:attachment;filename=".$file_name);
ob_start();
fseek($file,0);
while (!feof($file)) {
    set_time_limit(0);
    print (fread($file, 1024 * 8));

    ob_flush();
    flush();
}
fclose($file);