Include CakePHP function externally

I have a CakePHP app that functions like an index and admin panel for a bunch of webapps on my server. There is a prepended php file that should check whether a user is allowed to use a certain app.

This verification is done by a function/action of a controller of the index-app. So I would like my prepend.php to call this function and process the result. How could I do this?

Normally I would use include() or require() but since CakePHP actions are not routed as php files, this does not work. Is there another way? Or is it possible to have an action as a normal php file that can be included?

Thanks in advance!

EDIT: I should mention that file_get_contents([url]) works for an action, but it seems a little slow and I need the session to be available, which is not passed in that way.