I'm doing a bit of guess work here based on this failed unit test. Seems that the test uses usleep . Also i noticed (on my own machine) phpunit launches a ton of parallel processes. Take on top of that we don't know for sure what appveyor is running on 1 CPU. So that means a lot of processes are in contention for CPU time. So i guess that could be a cause here. If it is, below are some relevant links

https://github.com/php/php-src/blob/250938e2d35fc54161a18167b7901c5e3b574371/win32/time.c#L101-L113

https://msdn.microsoft.com/en-us/library/windows/desktop/ms687032%28v=vs.85%29.aspx

Note that a ready thread is not guaranteed to run immediately. Consequently, the thread may not run until some time after the sleep interval elapses. For more information, see Scheduling Priorities.

https://msdn.microsoft.com/en-us/library/windows/desktop/ms686298%28v=vs.85%29.aspx