yes, converting the code to create an array of the coupon codes will cause the server behavior to not show in the server behaviors panel
The idea is that by adding  to the end of the session variables name, it is now creating an array.
The custom trigger code to use would be:
isset($_SESSION["PromoCode"]) && in_array("12345",$_SESSION["PromoCode"])
This custom trigger is a replacement of the trigger that is used in the tutorial.