In his latest post Joe Watkins talks about mocking PHP. No, not making fun of the language but rather mocking internal PHP functions and methods as a part of unit testing your application.
I work on a vast PHP code base, it is 3M LOC of PHP alone. It's somewhere between legacy and modern, work is ongoing. [...] When I joined the current project there were many many tests, they relied upon the kind of unholy magic that runkit allows you to perform, for the most part this worked okay for a while. However, runkit inexplicably caused many of the tests to fault, either at shutdown, or at random.
[...] So we were in a bit of a jam, I've always found runkit to be quite awkward, and now I'm staring its source code in the face knowing it represents a road block to my goal of running the latest stable versions of PHP, with the first decent optimizer that ever existed for Zend. I tackled the problem with code, code which I was allowed by my gracious employer to open source (the uopz extension).
He goes on to talk about what the actual root problem he was trying to solve was (dodging code with built-in functions), the "obvious" way to solve it using runkit or the more modern solution that uses the uopz extension. He provides an example of it in use mocking the fopen function with a "uopz_function" wrapper.