Php noob question

I have noticed that the homepage for Talkfreelance ends in index.php?

What is the question mark for?

I am a total php noob and are learning the basics.

I do remember that ?xyz does a server refresh the same as hitting ctrl+F5

vBulletin uses a session id, it's a unique ID given to a visitor and on first visit and you will see something like this behind the URL of every link page.php?lkjsdf90dsfjs4ff

Because vBulletin stores the session id in a variable they have added a '?' behind most navigation links with the session id behind it.

Something like this: page.php?$session_id
So, when there is no session id it just shows page.php?


So basicly the question mark does nothing

See Query String (Wikipedia)

Thanks Bas and Salathe, I thought it might have had something to with helping SEO or something.