PHP Last Modified标头未发送?

所以我在PHP文件的最上面有这样一个:

<?php
// Set headers
header('Content-Type: text/html; charset=UTF-8');
header('Content-Style-Type: text/css');
header('Content-Script-Type: application/javascript');
header('HTTP/1.1 200 OK');
header('Content-language: en-US');
header('X-Powered-By: ');
header_remove('X-Powered-By');
header('Last-Modified: Tue, 01 Jan 2013 00:00:00 GMT');
header('Cache-Control: no-store, no-cache, max-age=0, must-revalidate');
header('Pragma: no-store, no-cache, max-age=0, must-revalidate');
header('Expires: Tue, 01 Jan 2013 00:00:00 GMT');
?>

因此,当我在站点URL上查看Firefox控制台的NET选项卡时,我看不到LastModified标头。有什么建议吗?