Many phone companies allow you to send text messages to phones from email.
For Verizon, to send a text message to the number 123-456-7569, you would simply send an email to firstname.lastname@example.org
So you would just write a basic contact form in HTML, hook it up with PHP to do the processing once they click Submit, and then use PHP's mail function to send an email with the contents of your form to your phone.
I run a small ecommerce site and I set it up with PayPal's IPN so that I get a text message notification whenever somebody buys something from my store with the items bought and the total price. It is very convenient.