I'm not referring to per-user sorts of things. You can't cache those locally in a sane way in any language. I'm talking about what would usually be memoizable sorts of things.

There are memoization libraries for PHP but really it's just another cache implementation. Put it in RAM, put it on disk, put it in memcached or redis, I don't really see too much a difference between $cache->set($key, $value) in PHP vs cache[key] = value in node for instance.