Manual:$wgFileCacheDirectory

Details
This is the directory where cached page will be saved when using file caching.

If you switch on server-side file caching $wgUseFileCache = true;, 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: $wgFileCacheDirectory = "/tmp/yourcache";

...but you can specify relative to other directory path variables e.g.: $wgFileCacheDirectory = "{$wgUploadDirectory}/cache"

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

You must create the directory and ensure it is writeable (by the php user). This is even true if you leave it as the default (You must create the "images/cache" directory).

If you are hosting multiple Wikis from a shared source you must ensure each has its own cache directory to avoid miss-serving pages with the same name (the Main_Page, for example).

See also File cache