I know that PHP is usually for form submission and things like that, but is it possible to make other applications like anonymous message boards, multiplayer games, and more backend apps?
I know quite a lot more can be accomplished with other backend languages like node, express, python, etc; But I was just wondering.
You can achieve the same things with PHP as you can any of the other two languages you mention, it’s a general-purpose language.
But even under the banner “general purpose”, specific languages/runtimes are designed for different things, they have different tradeoffs. PHP will work great for message boards. Re games, games are client-side things, not backend. But for game servers, it would work absolutely fine for turn based multiplayer games (as would anything else). For real-time, it’s not built to do that; you can finagle it, but out of the three languages you mention, it’s the least suited (and none of those languages are really terribly suited to it, especially w/r/t physics).