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.
Look at the "AcceptPathInfo" directive used in Apache (directly in httpd.conf or via .htaccess). It is a long time since I have used PHP / Apache, and even longer since I did this kind of url prettification, but it is essentially a way of passing extra data into index.php without the "normal" html syntax of ?var1=foo&var2=bar

Note, when I did it, I set up a script called 'dynamic' (or similar) as being the default, PHP parsed handler, and made my URLs look like <http://hostname/dynamic/value1/value2>

HTH