How to track a variable from the URL

[plk]

I'd like to be able to track a variable that is sent through the URL through the users session on the forum.

For example, say the user access the forum at URL:
www.myforum.com/boards?id=abc

I would like the "id=abc" to be carried throughout the forum session.

I think this may have something to do with "forumdisplay.php" but I've tried adding code these and although it adds the "&id=" it doesn't seem to be able to extract the id from the URL. I thought just using $id would be good enough but this doesn't work.

Any ideas would be greatly appreciated.

Thanks!