include troubles

[wanno]

I have a quit difficult problem.

In my forum I use an include file to have the same header as the rest of the site.

This include creates dynamic the code.
I do this with a url check:
$filename = $_SERVER['REQUEST_URI'];

If the include is included in the forum it creates code A
if the include is included in the rest of the website it creates code B

My problem is that REQUEST-URI gives always the same URL (because the include file is still the same).

How can I know the forum url and give it to the include?

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When I use
$filename = $_SERVER['REQUEST_URI'];
in an include in my website it gives the correct url of the location bar in the browser.

When I use
$filename = $_SERVER['REQUEST_URI'];
in an include in my forum it gives the wrong URL, that of the include and not the url in the location bar in the browser.

Why is this?
Can anyone help me how to retrieve the URL in the location bar?