php 下载文件

$file_name = 'a.doc';
header ( 'Content-Description: File Transfer' );
header('Content-Type: application/octet-stream;charset=UTF-8');
header ( 'Content-Disposition: attachment; filename=' . $file_name); 
header ( 'Content-Transfer-Encoding: binary' );
header ( 'Expires: 0' );
header ( 'Cache-Control: must-revalidate' );
header ( 'Pragma: public' );
header ( 'Content-Length: '  .  filesize ( $file_name ));
ob_clean ();
flush ();
readfile ( $file_name ); exit;