Basically, there isn't any structure either to the files or to their code, it's just sort of strewn about as is. Every php file that didn't come from an external source is in the root and there is a little organization for other files types, but not much. Most of the code isn't indented or readable.
I'm not trying to say it doesn't work, I've seen it work, it's just that I don't think anybody else could try to get a handle on what it does (and I don't want to sound like I'm ragging on the guy who wrote it, he told me that he really doesn't do much coding).
Basically, my question is should I back out, or should I try to press onward? Does anyone have experience with working on a big-old-system that needs to be completely updated? Any thoughts?
Additional Information: No, I don't have a contract yet, I've just looked at it thus far. My own feelings are that if I decide to press onward I'll end up rewriting the entire system (which is 113 php files in the root, plus several others located elsewhere in the system), and I don't know if I really want to continue, it's just that I feel like I said I would try to help out.