Wikimedia Bugzilla is closed!

In my mediawiki install, after submiting the login form, a banner about cookies
disabled was shown. Nevertheless the user was logged in.

I found that, when session.auto_start (php.ini) is on, mediawiki is unable to
change session_name() (in setup.php) and the cookie checking function fails
(because it checks for the cookie with the name when the one in php.ini is used).

The solution is to disable session.auto_start in php.ini (this could be checked
in the installation script).