PHPUnit has had built-in support for creating test doubles for many years. This implementation was originally inspired by the first generation of mocking frameworks for Java. Since then mocking frameworks have evolved. Modern mocking frameworks are more intuitive to use, lead to more readable code, and may even allow for a clear separation of a test double's configuration and the actual test double object itself.
Like many users of PHPUnit I am not satisfied with the API of PHPUnit's own mocking framework. This dissatisfaction has lead to the development of alternative mocking frameworks for PHP such as Mockery, Phake, or Prophecy. If I were to create a new mocking framework today it would probably look a lot like Prophecy. Which is why PHPUnit 4.5 introduced out-of-the-box support for it.
He gets into some of the basics of the Prophecy tool and how it handles mocking differently than the current internal mocking PHPUnit provides. Some code examples are included showing dummies, stubs and mocks with an example of the output when some of the "predictions" have failed.