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: 
Often, it is also done using a .htaccess file and mod\_rewrite.

See [this article](http://www.workingwith.me.uk/articles/scripting/mod_rewrite) for more info.
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).