Manual:$wgSharedDB

Details
The name of the shared database.

Shared database for multiple wikis. Commonly used for storing a user table for single sign-on (actually not real SSO, since the user may still have to log in into each wiki separately, but at least registration has to be done only once). The server for this database must be the same as for the main database. From 1.13 on you may use  to specify a separate prefix for this database.

See for usage information.