Get Variable from Header into Post?

Hello all,

I’m trying to use a GET variable and dynamically include it into a post. I have the PHP_EXEC plugin installed so I can run php code in my post. However, I put the GET code in the header and it won’t include in the post. The code is as follows:

In the header.php:

<?
$tid = $_GET['tid'];
if ($tid == "") {
	$tid = 'none';
}

and in the post:

<?php echo $tid ?>

Any ideas?