Looking at your code, if you're just trying to include an external page to pull layout elements (like what I have here) you can simply use a PHP include like:
Code
<?php include($_SERVER["DOCUMENT_ROOT"] ."/includes/header.forum.inc.php"); ?>

If I'm reading your code correctly you might be trying to include one header for two sites which may belong to different users on the machine, which could have any number of problems (such as permissions on the server); I'd advise you keep any includes local to every user, versus attempting to use only one file for various user accounts.

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