Manual:$wgRedirectOnLogin

Details
$wgRedirectOnLogin uses the URL parameter to redirect users to a specific page when they log in.

Specify the name of a page in the wiki where they should be sent automatically when they log in, for example: ' Main Page ', or ' Special:MyPage '. Setting this option to null will make the user be redirected back to the page they came from.

Examples
This will redirect users to 'Special:MyPage', i.e. the respective user's user page.