Manual:Central ID

The central ID identifies users across wikis. On single-wiki installations it is the same as the user ID; on wiki farms which use some kind of single sign-on, it's probably assigned by the authentication extension. Site admins can configure it by setting $wgCentralIdLookupProvider.

Any code that interacts with data that would benefit from being available cross-wiki (e.g. authentication and authorization details, cross-wiki blocks) should, instead of storing local user IDs, use the class which can convert between local usernames and central IDs. Single sign-on extensions should implement a subclass of CentralIdLookup and register it in $wgCentralIdLookupProviders.

For an example of a single sign-on extension providing central IDs, see  in Extension:CentralAuth. For an example of code making use of central IDs, see.