If you have files such as hello.php and you want users to be able to go to yoursite.com/hello, use the following code:

RewriteCond %{SCRIPT_FILENAME} !-d
RewriteRule ^([^.]+)$ $1.php [NC,L]

A request to http://yoursite/asdf will try and use the file /asdf.php.

Don't forget that you will probably need to enable the rewrite engine for these rules to work. Click the button below to find out how to do this.