PS: If someone wants to give a hand to find the regression... I can
explain how to reproduce.
The easiest way is to use a tool to help reducing the test case automatically.
I don't know about any specific tool for reducing PHP files, but in the past I've used delta (http://delta.tigris.org/), which is a reducer for C/C++ and it works quite well with PHP.
You have to do two things in order it to work: disable topformflat (because it doesn't understand PHP), and inline the included files after the first reduction step, and keep inlining the include()'s that survive to a reduce step.
Nevertheless, it's a fairly trivial process.
Nuno