Hello!
WebRequest::getPathInfo() still depends on PHP bug 31892 fixed 6 years ago. I.e. WebRequest uses REQUEST_URI instead of "mangled" PATH_INFO which is not "mangled" since PHP 5.2.4. Yeah, Apache still replaces multiple /// with single /, but afaik it's done for REQUEST_URI as well as PATH_INFO. Maybe that part of the code should be removed?
Also I don't understand the need for PathRouter - my IMHO is that it's just an unnecessary sophistication. As I understand EVERYTHING worked without it and there is no feature in MediaWiki which depends on a router. Am I correct?