Manual:$wgProto/en

Details
This value is set automatically when the value is determined for if the server where the MediaWiki installation resides is a secure server.

Note that prior to 1.6.0 this setting was only used temporarily to create, and was unset afterwards, but from 1.6.0 onwards it remains set throughout, and is used to calculate the default value of.

Value is set by includes/DefaultSettings in a "best guess" pattern. If you want to change it, you need to overwrite other related values as well or modify DefaultSettings.php.