Padraic Brady has made a post about some of the mutation testing thoughts running through his head for the Behaviour-Driven Development framework he's been working on - PHPSpec.

Mutation Testing is like testing, for tests. The idea is actually quite simple. Mutation testing adds small changes to your source code on the assumption that changing something, will most likely break it, which in turn means at least one test/spec should fail. [...] The reason why it's useful it that it gets around the problem of being over reliant on code coverage - just because some code executes when running tests, doesn't prove the tests will detect problems with it!

He wants to create a way that doesn't rely on external dependencies and is looking at working with the tokenizer functions native to PHP. He includes two examples of the way he's thinking - an example of simple token output and a way to go "backwards" to recreate the original source code.