Run rbl script once per user
 

I have a script that I'd like to execute ONLY once when a user accesses my forum. Because this script checks a rbl database I don't want to have it run every time a user changes pages. Where is the best place to call the script.

require_once('./rbl.php');

What does "once" mean in the context of a stateless protocol as HTTP?
Once per page access?
Once per vBulletin Session?
Once per Browser Session?
Once per "User uses his PC" Session (how do you define session start/end in this case? Is ti still the same session if I leave for 10 minutes to get smth. to eat?)?
Or smth. completely different?

Good questions all of them ;-)

My goal is to have the script run once when the user accesses the main forum page but not again until perhaps the session expires to reduce dns lookups. I'm going to assume a browser session is until the browser is closed and vBulletin Session is until the cleanup occurs. Of course I really don't know I'm just guessing. Perhap you can suggest some options.