On the php|architect blog today John Mertic looks at a validation library that's been created to filter and handle user input for your application without too much effort on your part - Validation from a group called Respect.
It's simply called Validation, and it is a very simple and straightforward PHP 5.3 based validation library. The goal for them is to make validation easy and painless, using clever constructs like chaining and enabling developers to create reuseable components.
He shows a sample of how it all works by creating a validator that checks to be sure a given value is numeric. He also shows how you can chain the validation methods to create more complicated rules and even check for a set of value types (ex. numbers 1-10 or the string "NO"). You can find the code for Validate on their github repository.