Tiny text script thing-a-ma-bob

[Useless]

Anyone know of a light script that can overwrite a single text file from a form? In other words, I want to be able to paste a bunch of text into a <textarea> of a form, hit submit, and have it overwrite what ever info is on data.txt, for example.

I can probably reverse engineer some bigger script that stores data on flatfiles and figure out how it's done, but if anyone knows of a small script that does that, I'd be eternally grateful, or at least until I fuck it all up.