My motivation was that PHP powers almost every website these days, and admins may not think about adding the extension here. It's common enough that it should be included. It's far worse not to include it.

If we don't include all dynamic file extensions here, then the browser won't receive any Cache-Control header telling it not to cache those files, which in turn means that the browser will try to use heuristics such as "cache it for 10% of the time that has elapsed since Last-Modified" and could cause undesired results.

If anything, we should add more languages than just PHP.