Passing session in request to external script - how?

[Spinball]

Hi folks,
I have my own ad banner serving script which in showthread does a

PHP Code:

$userip = $_SERVER['REMOTE_ADDR'];
$headerbanner = file_get_contents("http://www.mydomain.com/ad_serve.php?zoneid=" . $forumid . "0&ip=" . $userip) . "<P></P>";
$footerbanner = file_get_contents("http://www.mydomain.com/ad_serve.php?zoneid=" . $forumid . "1&ip=" . $userip) . "<P></P>"; 


Which serves different banners at the top and bottom of the SHOWTHREAD template depending on the forum. It also records every banner shown in a table and when it shows a new banner, it picks the one in the pool for that location which has been seen least by that person (i.e. ip address).
It works fine, but I have a potential customer who only wants to show his banner once per session.
If I pass the vB session as part of the query string, what code would I have to use and how big a field would I need to store in the database (would it be varchar 32)?
TIA

Ok I figured out that it needs to be ?" . $session['sessionurl'] . but when I put this into showthread, it doesn't have a value. Which is very curious.
Can anyone help, please?