Manual:$wgUpgradeKey

Details
When overridden in LocalSettings.php, this setting allows to run upgrades from web interface. This variable's value serves as password protecting the upgrade script.

Installer in 1.17+ adds this variable to LocalSettings.php and fills it with a random password, but leaves it commented out. If you're intending to use web-based upgrades, uncomment it (or add it if your LocalSettings.php was created by an earlier version of MedaWiki).