[PHP] Starting session

[Rosen]

Hi,
I'm creating a WEB portal  with PHP.
I want to give possibility the users to register on my site and to use some
features on it.
I want to use sessions and to start session only if user login.
How can I understand ot my other pages on site, thath the user has logged in
my site.
i.e. to startsession only if user has logged in ?

Thanks,
Rosen Marinov