Everything worked until I decided to enable user authentication. Then I couldn´t login anymore (checked user and pass, restored pass from mysql cli, still no logon). Checking apache logs I discovered that the problem was the directory that PHP uses for storing authenticated sessions. It must be writable to the user that is running apache. In my case (running on Slackware 13.0) the fix was on the file /etc/httpd/php.ini comment the line session.save_path = "/var/lib/php" and uncomment session.save_path = "/tmp"
Info on this: http://bytes.com/topic/php/answers/1105 ... m-sessions
Hope this can help anyone!
Good luck!