Federico Cargnelutti has a helpful post to his site today for the unit testing/TDD crowd about checking the retuned value from a stub of an object in your tests. He's using the built-in mocking framework here, not something like Mockery.

State verification is used to ensure that after a method is run, the returned value of the SUT is as expected. Of course, you may need to use Stubs on a test double or a real object to tell the object to return a value in response to a given message. [...] In PHP, for example, you dynamically type the return value within the body of the method. This means that PHP mocking libraries cannot check the type of the return value and provide guarantees about what is being verified. This leads to the awkward situation where a refactoring may change the SUT behaviour and leave a stub broken but with passing tests.

He gives an example of a few classes - a Presenter and Collaborator - and a test that mocks out the Collaborator instance, calling a "getStories" method on it. He shows a situation where all tests pass in the initial version, but after some changes to the return type, a test that should fail doesn't. His solution for the issue revolves around DocBlock annotations and the Return Value instead of the built-in mock object return method.