Attila Szabo has come up with a class that makes sending SMS messages from your site simple (through the smsBug gateway).

I wrote a simple PHP class for sending SMS messages through smsBug gateway. It can send a single SMS message and receive the number of credits left (see the UML diagram of the classes below). You need to include two classes into your PHP code.

He includes an example of it in use (sending a single SMS message to a phone) as well as a demo of it in action and a link to the download where you can grab the class for yourself.