I commented it for clarification.

This way I have full control over Search Engine Friendly URLs in my PHP and can do whatever I need dynamically, instead of modifying .htaccess every time I need to add a module. main.php is my controller.

RewriteEngine On
# If the requested local file path is !NOT a regular file (-f) in the file system
RewriteCond %{REQUEST_FILENAME} !-f
# If the requested local file path is !NOT a directory (-d) in the file system
RewriteCond %{REQUEST_FILENAME} !-d
# Rewrite Any string with 0 characters or more and replace it with index.php
RewriteRule ^(.*) main.php [PT]

I use this approach on every site I create for several reasons:

1. Its very flexible.
2. Though flexible, its very strict and secure. If the URL does not match the regex, then it gets a 404. This is a good front to validate and protect against query string injection.
3. Control of the request/routing. I can specify 301/302 and even 404 if necessary and then point them accordingly.
4. Mapping URLS. I have re-build several sites and am able to re-route the old indexed URLS with a 302 to let search engines know the page has moved. This helps for those who have pages bookmarked or search engines who have pages cached. I can also map a page with two different patterns. IE (www.barbourbooks.com). The book detail page can be accessed with its title friendly URL, or by putting in the ISBN of the book directly (though they must match valid ISBNs, as well as be in our stock).
5. Programming language doesnt matter with this approach. I can map some pages to PHP, ASP, CFM, JSP, etc – or whatever I want (if you so choose) and not have to change the url structure or extensions. So a system switch from one programming language to another will not be affected.

Nice read!

Peace,
Nate