Joshua Thijssen has a recent post spotlighting a part of the Standard PHP Library (SPL) that implements that Traversable interface, the IteratorAggregate interface.

Together with its more famous brother "Iterator", they are currently the two only implementations of the "Traversable" interface, which is needed for objects so they can be used within a standard foreach() loop. But why and when should we use the iteratorAggregate?

He answers his question with an example - a book that contains chapters. With a normal iterator you'd have to define standard functions (like valid, rewind or key). Using the IteratorAggregate you can push items into an internal array (like chapters in a book) and call a "getIterator" method to get this set. He also takes it one step further and shows implementing the "Count" interface to make it easier to get a total count of the items in the iterator. Sample code is included to help clarify.