As many do I have a config.php file in the root of a web app that I want to include in almost every other php file. So most of them have a line like:
require_on
The path of the PHP file requested in the original GET or POST is essentially the 'working directory' of that script. Any "included" or "required" scripts will inherit that as their working directory as well.
I will either use absolute paths in require statements or modify PHP's include_path to include any path in my app I may want to use to save me the extra typing. You'll find that in php.ini.
include_path = ".:/list/of/paths/:/another/path/:/and/another/one"
I don't know if it'll help you out in this particular instance but the magical constants like FILE and DIR can come in handy if you ever need to know the path a particular file is running in.
http://us2.php.net/manual/en/language.constants.predefined.php