Mike Weiner has put together an article for PHPBuilder.com that's posted today showing a bit more object-oriented solution to validating the user submitted data in your forms:

Whether it is for database submission, emailing, or for some other purpose, forms represent the primary means of enabling a user to send data to an application. As a result, it is important to have control over the data collected by your forms, which will aid in the creation of streamlined, error-free applications.

He lays out the validation method in a graphic going with a generic validation class that then calls the various validation methods (like phone or email) as needed. The "validation set" manages which validations are to be run on which form elements. Example code for each of the bits making up the structure and the implementation are provided.