Manual:$wgForceHTTPS/en

Details
If this setting true, when an insecure HTTP request is received, always redirect to HTTPS. This overrides and disables the preferhttps user preference, and it overrides and the  hook.

may be either https or protocol-relative. If starts with "http://", an exception will be thrown.

If a reverse proxy or CDN is used to forward requests from HTTPS to HTTP, the request header "X-Forwarded-Proto: https" should be sent to suppress the redirect.

In addition to setting this to true, for optimal security, the webserver should also be configured to send $1 response headers.