Manual:$wgUsePathInfo

Details
Whether to use 'pretty' URLs, e.g.

Note: These often break when PHP is set up in CGI mode. PATH_INFO *may* be correct if cgi.fix_pathinfo is set, but then again it may not; lighttpd converts incoming path data to lowercase on systems with case-insensitive filesystems, and there have been reports of problems on Apache as well. To be safe we'll continue to keep it off by default in these instances.

Override this to false if $_SERVER['PATH_INFO'] contains unexpectedly incorrect garbage, or to true if it is really correct.

Default value
Note that prior to 1.9.0, only 'cgi' was checked for.