Question: Some sites (for example, certain implementations of mediawiki,) have a uri format of `http://example.com/index.php/SOME_PAGE`. How is this done? Is index.php just a directory on the server that happens to be named like a php file and that actually contains a file named `SOME_PAGE`? If so, why name it like that? If not (i.e. it actually *does* represent some sort of legitimate PHP file), then what would the contents of the look like?

Answer: 
This feature is called path info (see [Apache directive `AcceptPathInfo`](http://httpd.apache.org/docs/2.2/mod/core.html#acceptpathinfo)).

When enabled, Apache writes the rest of the requested URI path that’s already mapped to an existing file or directory in the `PATH_INFO` environment variable. So if `/index.php/SOME_PAGE` is requested and `/index.php` is an existing file, the request is mapped to that file and the rest of the path after `/index.php` (`/SOME_PAGE`) is then available in the `PATH_INFO` variable.
Those sites rewrite incoming requests to point to the actual resources on the server using web server modules such as apache's [mod\_rewrite](http://httpd.apache.org/docs/1.3/mod/mod_rewrite.html).