This is a live demonstration of an XML based form validation that use the same rules to validate on both the client and the server.
vForm use Javascript to validate input on-the-fly, before it is submitted, and PHP on the server-side to ensure that the input is valid. What makes vForm special is that it allows you to only define the validation rules and error messages in one place and them use them on both the client and the server. vForm also comes with a bunch of predefined validation patterns and also allows you to add your own.
Visit my weblog for more information on vForm.