On NetTuts.com today there's a new tutorial showing you how to send text messages via SMS directly from your script to the user's cell phone.
Text messaging has become extremely widespread throughout the world — to the point where an increasing number of web applications have integrated SMS to notify users of events, sales or coupons directly through their mobile devices. In this tutorial, we will cover the fundamentals of sending text messages with PHP.
Their method "cheats" a little bit and uses an interface many of the cell phone providers offer to their subscribers - a link from an email address to SMS. Most providers have their domains you can send the email to and have it automatically forwarded to the user's cell as a text. Thankfully, this requires nothing more than the mail function to accomplish. They include the sample code and HTML+CSS to make a basic form as well as mention PHPMailer as a more powerful mailing alternative.