Manual:$wgFileCacheDirectory

Details
If you switch on server-side file caching  , then you can use this variable to specify where the cached pages will be saved on your server filesystem.

It's an absolute path, so you could specify somewhere under /tmp for example:

...but you can specify this absolute path in relation to other directory path variables, e.g.:

This specifies a "cache" directory under the image uploads directory (normally called "images"). This is the default setting in fact.