On the Web Builder Zone (a part of DZone.com) there's a recent post from Giorgio Sironi reviewing the Mockery library, a mock object framework created by Padraic Brady.

Mockery is a mock object framework (more properly Test Double framework) from @padraicb, independent from testing frameworks like PHPUnit. It can be used to quickly prepare Mocks, Stubs and other Test Doubles to use inside your unit tests. I've tried Mockery via a PEAR installation and I must say its expressive power is higher than that of PHPUnit mocking system. However, it may be too powerful for effective usage.

He talks about some of the features that are in Mockery that aren't in PHPUnit like alternative expectations, recording of the expected calls and the mocking of non-existent methods. He notes that it can be too overwhelming at times, though, with so many features that may or may not be useful for the large majority of testers. He includes some sample code showing a set of five tests on a simple class implementing an interface.