Error after fresh install of 1.22.0
There seems to be a notice appearing on different installations of 1.22.0 where PHP gives out a warning on the login page. There is a thread on the support desk that brings the issue in conjunction with $wgCommandLineMode.
A suggested workaround for the error is to set $wgCommandLineMode to true in order to suppress the notice. However, this is discouraged on this page.