It's a nice idea, but it's been tried and abandoned more than once. A file can only be written to by one script at a time, and while it's being written, no other scripts may read from it.
But more importantly, you'll have to write new code for every function. Basically you're re-inventing the wheel.
MySQL is very small and simple, and saves you month/years of development.
A forum, a FAQ, email notification, what else do you need?