原生php文件下载功能

<?php
header ( 'Content-Description: File Transfer' ); 
header ( 'Content-Type: application/octet-stream' ); 
header ( 'Content-Disposition: attachment; filename=' .$filename); 
header ( 'Content-Transfer-Encoding: binary' ); 
header ( 'Expires: 0' ); 
header ( 'Cache-Control: must-revalidate' ); 
header ( 'Pragma: public' ); 
header ( 'Content-Length: ' . filesize ( $file )); 
readfile($filename);