Alright, I have this CMS I developed. I installed it without any issues on the production server, then I sent the client the admin panel (C++/Qt application) and the installation package for the development server. The code was as far as I can determine extracted correctly on the development webserver and the installation script was accessed without any troubles. That's where the issues start.
The client is running a XAMPP installation (tried latest and 1.7.1). When attempting to run the installation script, the configuration files containing essential variables are included normally (verified using an additional test script I had the client run, using require instead of include), the only issue is that the variables from the included script are then not available in the including script. Isset() reveals that they aren't even declared, let alone defined.
The installation script itself is a simple script, with no functions or classes, just straight-forward code running in the global namespace. It has worked fine for me on my local servers and various remote servers. The CMS has been installed on nearly a dozen individual servers by now without issues. The only thing different in this case as far as I can determine is the use of XAMPP instead of a regular Apache + PHP combo.
Any thoughts/feedback would be most welcome.