[TYPO3] Custom PHP-Code but not in template

[Rafal Markut]

Hi,

I have to make a PHP script which has to loggin users in the "background".
We have Single-Sign-On system and it should be connected with with typo3 
application.

The typo3 application has login screen. We want that if someone loggs into 
our Single-Sign-On and then he clicks some link then automaticly should be 
logged into typo3.

I wrote PHP-code which check if the user has logged per Single-Sign-On. Now 
I have to write code which logs-in concrete user in typo3 app.
This cannot be in the template (redirection won't work). I think that 
writting only for that module or extension wouldn't be so good idea - but 
correct me if I'm wrong.

Maybe should I create a simple file with whole php-code (SSO authorization, 
then loggin in typo3 and redirection to suitable typo3 page) - but then this 
PHP-code need to work (i.e. have access) with typo3 API.
So I think that these PHP-Code needs to have some initialization-code, which 
somewhow connects with typo3.
I was looking in the dokumentation, but I didn't find anyting really useful 
for me.

How could I solve this problem ?
Are there any exmaples of such cases ?

Typo version 3.7.0

with best regards,
Rafal