The form button is just like any other form button, nothing special. Catch the POST on the php side of things, and use session_destroy(); to kill the session data entirely.
See this guide for info about forms and post if you're hazy on the subject: http://www.tizag.com/phpT/postget.php and this http://www.tizag.com/phpT/phpsessions.php for info about sessions
More info about forms and PHP and how to work with the data from the form: http://www.tizag.com/phpT/forms.php
Example:
Order.html:
<html><body>
<h4>Tizag Art Supply Order Form</h4>
<form action="process.php" method="post">
<input type="submit" />
</form>
</body></html>
process.php:
<html><body>
<?php
session_destroy();
?>
</body></html>
It's cheesy...does this help?