all external URLs look like 'module/action?key1=param1'. No customization possible--but it's fast. The difference is that the first uses PHP's GET, and the second uses PATH_INFO.
I've seen PATH_INFO several times,but still don't know what exactly it is?
Actually, PATH_INFO is related to the Apache Web Server serving PHP pages and not PHP per-say.
PATH_INFO is an environment variable set by Apache when the AcceptPathInfo directive is turned on. It will contain trailing pathname information that follows an actual filename (or non-existent file in an existing directory) will be accepted or rejected. Environment variables are then passed on to the Apache/CGI module in charge of rendering the page.
The variable is accessible in PHP using $_SERVER['PATH_INFO'].
For example, assume the location /test/ points to a directory that contains only the single file here.html. Then requests for /test/here.html/more and /test/nothere.html/more both collect /more as PATH_INFO.