How to include a php file in typo3

From your typo3 backend, choose from the left menu: Web/Template/Setup section or Web /Template/ Your extension / Setup
And add the following line (change the path to your corresponding file) in the 'setup' section of your template record:

config.includeLibrary = fileadmin/lib/file.inc.php

You can read more here:
http://typo3.org/documentation/document-library/references/doc_core_tsref/current/