Manual:$wgCanonicalServer

Details
Server name with protocol (HTTP or HTTPS) to use for links to the wiki from external locations, such as IRC feeds and email notifications. Must be fully qualified, even if $wgServer is protocol-relative.

You can often ignore this variable. However, if you have set $wgServer to a protocol-relative URL (for example,  ) and you would like links to your wiki to use HTTPS, set this variable to the equivalent HTTPS URL (for example,  ).