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 $wgSecureLogin and the CanIPUseHTTPS hook.

$wgServer may be either https or protocol-relative. If $wgServer 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 Strict-Transport-Security response headers.