Sending SMS using PHP

Im looking for a way to send SMS notifications using PHP.

Does anyone have any experience with this?

Googling around the most solutions to this are actually email to sms which has two draw backs as far as I can see. First you need to know the recipients network and second the recipient will get charged for receiving it.

Basically I have a paid service that I wish to add SMS alerts to. An API would be great and dont mind paying for it as long as its reasonable.

Any suggestions?