Manual:$wgUsePathInfo/en

Details
Whether to use 'pretty' URLs (or Short URL), e.g.

These often break when PHP is set up in CGI mode. 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  contains unexpectedly incorrect garbage, or to true if it is really correct.

The default  will be set based on this value at runtime, but if you have customized it, having this incorrectly set to true can cause redirect loops when "pretty URLs" are used.