There are a number of ways to make “clean URLs” work on your site, but this one is pretty straight forward. It allows you to access /any-page.php by simply going to /any-page. Just place the following into your .htaccess file (and make sure that mod_rewrite is enabled):
RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}\.php -f
RewriteRule ^(.*)$ $1.php
The nice thing about this is that it doesn’t affect querystrings. With this solution, both of these URLs are effectively the same:
http://example.com/page.php?id=123 http://example.com/page?id=123
Of course, the caveat is that you don’t have “clean querystrings”, but it’s a reasonable trade-off between “clean” and configuration.