Where are $usercache and $postache are defined

[Opserty]

$usercache is a temporary variable which stores/caches data from fetched users. It means that if fetch_userinfo() is used to fetch the data of a user which has been previously fetched, then instead of querying the database, the cache is returned. I'm not sure why you want to know about this one, its just really to optimise the fetch_userinfo() (and others) function.

I presume $postcache is something similar. Unless someone else has other ideas.