At some point, many php developers turn to the pcntl functions in php to write a daemon, or server, or simulate threading. But how do you unit test this with complete code coverage? [...] We need to engage some black arts php extensions to make this happen. An installation guide follows, and the post ends with a complete listing of the unit test.

He uses the test_helpers extension (as provided by Sebastian Bergmann) and Runkit to allow the test to define new methods copying the current pcntl methods and mocks up the responses. Tests are included to check the parent of a process, checking the children of a process and testing that a fork could be made. Hes's even included visual proof of this working.

While at the OReilly Open Source Convention in 2009, we were asked about how we pulled this off. So, we are releasing the two scripts that handle the forking and some instructions on how we use them. [...] These are the scripts we use to manage long running processes. They work great for us on Linux. They will not run on Windows at all. We also never had any trouble running them on Mac OS X.

The two scripts (that you can grab on their developer site) show two methods for running child processes and how to pass off requests to each one to be handled in turn.