After a successfull user login, and before session (or other internal variable) setting up the user is successfully authentified, I want to call a php script (which, based on its results, may affect theses variables saying user is or not logged).
Can you make me clear about which source code file, function, variable, may I modify? I don't know about phpbb software architecture and there are a lot of files...
It is not recommended that you edit core code but that you use extensions, which as you have posted in the Extension Writers Discussion form is what I am guessing that you are trying to do.
I would suggest that you start by looking at the core.login_box_redirect event - or possibly one of the other events in that function.
David Remember: You only know what you know and - you don't know what you don't know!
I now no longer support any of my extensions but they will start to become available here