1) I think your trigger may be wrong... I'd update line 275 to:
if (isset($_SESSION['PromoCode']) && $_SESSION['PromoCode'] != "") // Trigger
2) You are setting the value from the recordset, but it appears you are using the wrong parameter to filter the recordset.
Lines 43-45 should be:
if (isset($_SESSION['PromoCode'])) {
$colname_discounts = $_SESSION['PromoCode'];
}
You have $_SERVER on those lines.