PHP在线生成txt文件下载

<?php
$str = 'txt内容';
$filename = '文档名称.txt';
header("Content-type: text/plain");
header("Accept-Ranges: bytes");
header("Content-Disposition: attachment; filename=".$filename);
header("Cache-Control: must-revalidate, post-check=0,pre-check=0" );
header("Pragma: no-cache" );
header("Expires: 0" );
exit($str);
?>