In that first part, I implemented the ORM’s data access and mapping layers. And as you’ll surely recall, the entire implementation process was pretty straightforward and easy to follow. Of course, in its current state the ORM is still far from a fully-functional structure. We need to add some additional components to it, such as a domain model and the classes responsible for handling collections of entities (remember that the ORM relies heavily on the data mapper pattern to do its business properly).
He stays with his "simple blog" example and shows domain models (based on an abstract entity) for Entries, Comments and Authors. His containers extend the Countable, IteratorAggregate and ArrayAccess interfaces to give them some extra abilities.