Padraic Brady has posted about a library that he's developed to help with mutation testing in PHP - MutateMe. What's mutation testing? Padraic explains,

Mutation Testing is basically testing...for tests. It ensures that your tests are truly capable of detecting errors and problems with the source code. It does this by mutating the source code itself (using ext/runkit) in such a way that an error is created in the code. If your tests detect the error, all is well with the world. If your tests do not detect the error...well, you better add a new test that does.

He mentions how it compares to code coverage and goes through the entire process you'll need to get MutateMe up and running - the download (or PEAR install), creating the extension, using the command line tool and the results of an example run of the client against a few tests.