From a apllication point of view you should always interact with a ConnectionInterface typed object, but in fact it will alway be a ConnectionWrapper. Otherwise we wont have support for nested transaction, even when not using this new transaction helper.

The transaction helper api should support nested transactions, there will only work on the ConnectionWrapper level. It would also make sense to add the method to the ConnectionInterface and implement it on the wrapper. In the PdoConnection the method could just throw a exception? I am not sure though.. Any alternatives!?