Manual:$wgUpgradeKey/en

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

Installer in 1.17+ adds this variable to LocalSettings.php and fills it with a random 16-character password. If it is not set in LocalSettings.php, the upgrader will tell you what to set it to in order to authorize the upgrade process.

For security, do not set this to a guessable string. Use the value supplied by the install/upgrade process. To cause the upgrader to generate a new key, delete the old key from LocalSettings.php.