Noting the lack of a good, independent library for creating Mock Objects for unit testing PHP scripts, Padraic Brady (with assistance from Travis Swicegood) has come up with a standalone Mock Object and Stub framework - Mockery.
It's not tailored specifically to any one testing framework, rather it's an entirely separate framework with a discrete API. The idea is that you can use this framework within PHPUnit, or SimpleTest, or anything else really, without being forced to rely on the built-in support (if any) that test framework provides. Mockery was designed specifically to implement a form of Domain Specific Language (DSL). It makes extensive use of a fluent interface mixed with methods to approximate plain English.
He includes an overview of what Mock Objects and Stubs are and how to set them up with the Mockery tool. He finishes things off with a look at the Mockery and Mock Object Expectation APIs. You can grab this first release either as a PEAR package or from its github page.