Ulrich Kautz has recently taken a look at validation and sanitization of data in PHP applications. He talks about several different methods - both in core PHP and in various frameworks.
Validation and sanitization are extremely important topics, any developer should be aware of. Especially with powerful, modern frameworks, people seem to forget about the underlying concepts and wrongly assume it's already solved somehow. Correctly used and early on integrated, both play the central role in defending against attacks on your application.
He talks a bit about why you should care about the topic, some of the common issues/threats that could come up because of it and some general information on what validation and sanitization are. He looks at implementation with the filter extension and touches on functionality from Symfony 2, Laravel 3, CakePHP 2 and shares his own data filtering module with examples of its use.