When I was installing MediaWiki and reached mw-config/index.php?page=Welcome, it complained "No DB module". I went to install php5-mysql, then came back and reloaded the page. However the big red error box was still there. I tried several ways to restart the installation, including going to index.php, mw-config/index.php and clearing session cookies of the installer. No method except for the last worked but the last is really unfriendly. A newbie can be even misguided that the php-db environment is not correctly configured.