Using PHP Sessions in WP 2.3

I have a beautiful shopping cart that I wrote in PHP/MySQL and I want to integrate it into my WP site, but my sessions keep getting reset to NULL. Does anyone know why this is and what can be done to allow my sessions to run in WP? As I understand it, WP uses cookies but does not touch the session arrays at all, so why does it keep resetting my $PHPSESSID value to NULL?

On my main home page above the header call I have:

<?
session_start();
?>

and then on my shopping cart pages, above he header call, I have:

<?php
if (!$PHPSESSID)
{
header(“Location: index.php”);
exit;
}
session_start();
?>

So, what’s the deal?