We had just come across this problem with a particular website we are developing and believe that we know of the main cause as we have never received this error before and try to test most limits on development.
The Problem Code:
The Fixed Code:
$orderItem = $_SESSION["orderItem"];
$orderPrice = $_SESSION["orderPrice"];
Basiclly we have until now used the exact same name for a session variable in this way, But seems this was the cause of the error for us.
$SorderItem = $_SESSION["orderItem"];
$SorderPrice = $_SESSION["orderPrice"];
Hope this helps a few people around?