Manual:$wgAutoloadClasses

Details
This array maps class and interface names to PHP filenames. MediaWiki core's uses it to load those files.

Extensions (and skins) specify this class mapping in the  key of extension.json (or skin.json), for example from the BoilerPlate extension's :

Example prior to extension.json
Assume the extension NewExtension's files are in the directory extensions/NewExtension.

If NewExtension implements a class called NewClass in the file NewClass.php, then in its initialization file NewExtension/NewExtension.php it should add to  as follows: