I keep getting the following erros when adding something to the cart:

Notice: Use of undefined constant s_id - assumed 's_id' in /xxx/roster/addons/shopping/phpCart_manage.php on line 7

Notice: A session had already been started - ignoring session_start() in /xxx/roster/addons/shopping/functions_cart.php on line 9

Notice: Use of undefined constant s_id - assumed 's_id' in /xxx/roster/addons/shopping/functions_cart.php on line 10

Notice: Undefined index: HTTP_REFERER in /xxx/roster/addons/shopping/functions_cart.php on line 98

Please help?

Thanks!