Rafael Dohms has an interesting new post to his site today looking at a library he's developed (inspired by the Symfony Validation library) to help filtering values with rules defined in annotations.
PHP does not have native Annotations support, however many projects have been using doc blocks to add value and semantics to code, like PHPUnit, Doctrine and Symfony. The Doctrine did a really good job in making available a Annotation parser kit, which allows you to bring the power of annotations into you own project. This opens up a few possibilities.
He shows the current use of the Validation library with a sample check of a $name variable for "not blank". His tool, DMS, extends this functionality and gives you access to both custom filtering methods and standard PHP functions to filter the resulting variable value. He includes an example showing two variables with filters StripTags, StripNewlines and Trim as well as the code to execute the filtering. You can get the library either ready to integrate with Symfony/Doctrine or as a standalone tool.