DevShed has the second part of their "Building a PHP5 Form Processor" series posted today - Coding the Form Validator Module. It looks at how to construct a set of functions that will check the data inputted to ensure it exists and correct/valid.
In this second part of a three-part series, we take up the topic of server-side validation. By the time you finish this article, you'll have the general guidelines for how to build a form validating class. You'll use some PHP built-in introspection functions, along with regular expressions, to assist you in building this class.
They create several functions, including validateEmpty, validateInteger, validateNumber, validateRange, and validateEmail. Each one (obviously) serves their own purpose, and some are more complex that others (validating an integer versus a valid email address), but they walk you through each, showing you plenty of code and explainations along the way...