[Typo3] PHP Include in TypoScript

[James Walsh]

I am trying to convert an existing site to TYPO3.  
I need dynamic header menus as in my current site, and
I achieve this via PHP at the moment. All i want to do
is get Typoscript to call the php file like currently.

Can someone please tell me precisely what TypoScript
code I should use to get it to include HTML generated by
a PHP script(below). The code below is the php include 
code I am using on my current php website.

<?php include_once 'fileadmin/header.php'; ?>

As a newbie, I am drowning(!) in the TYPO3 documentation but still
can find this simple (you would think!) code .

Thanks
James