WebReference.com has posted a new tutorial looking at filtering data (user input or otherwise) in your PHP applications:

In this article, we will be exploring some of the many data filters that are offered by PHP. These functions are available since PHP 5, and no extension installation is required since it is part of PHP 5's core. We will also be looking at what data filtering is and why it should be used in web applications.

The tutorial covers the safe filtering of the superglobals (possible user input) and using functions like mysql_real_escape_string and the filter extension to do it. Some sample code is included to show you how to both filter and evaluate the incoming data to ensure it's valid.