While working in my environment I noticed the same thing on my server until I realized that $_SERVER['PATH_INFO'] existed *only* when the URL contains the reference to the index.php file, like this:

http://yoursite.com/index.php/this/is/path/info

If your URL is like this:

http://yoursite.com/controller/stuff/and/things

Then the $_SERVER['PATH_INFO'] key does not exist and you must use $_SERVER['REQUEST_URI'].