The forum was primarily about the “Penguin Client Library” (or “Penguin Client System”, I think they went back and forth about the name?), which allowed writing PHP scripts to interact with the game servers.

Why PHP? I think maybe it was originally so people could use it to make web forms where people could put in their username and password and it would e.g. give them whatever item, but that kind of cheat was blocked very quickly, and I think it just remained in PHP for historical reasons, so instead you had a bunch of people running PHP on their local machine to run a bot doing normal game actions (but combined in unusual ways). Or maybe it was just the language the devs were most comfortable with, idk.