One of PHP\'s double-edge sword is that you don\'t have to do any special function calls to get the variables out of the URL.
In your link:
http://someurl.com/forum/forumdisplay.php?s=&forumid=12
to get the forumid out of this in your PHP page, all you need to do is treat forumid as a variable.
For example, if all you wanted to do was print the forum id, you would just need to do something like this:
<?PHP
printf(\"$forumid\");
?>
and it would echo out \"12\"
I hope this all makes sense