Simon Willison - Easier form validation with PHP:

"FormML (for want of a better name) is never passed to the browser; instead it is processed by my PHP FormProcessor class before being displayed. This class strips out all of the FormML tags, and also applies logic to the rest of the form based on the information from the tags. Because the whole thing is XHTML, this can be relatively easily achieved using PHP's built in XML parser. The XML is modified on the fly to create the XHTML that is sent to the browser."