These are a few practical examples, with the XML that generates them and PHP that processes them. Also, for security concerns this does not display the PHP variables, however it will email you or display a message.
This is to show how to use this in an email form. It uses a regular expression to make sure that it is a valid email address, then sends the message to that email address. Source XML
This is a demonstration of a very simple login form. User name is "user" and the password is "password". Source XML
This is a demonstration of a very simple registration form. Source XML