PHPMaster.com has a new tutorial posted today (by George Fekete) about preventing cross-site scripting attacks in your PHP-based applications.
Unfortunately, cross-site scripting attacks occurs mostly, because developers are failing to deliver secure code. Every PHP programmer has the responsibility to understand how attacks can be carried out against their PHP scripts to exploit possible security vulnerabilities. Reading this article, you'll find out more about cross-site scripting attacks and how to prevent them in your code.
Included in the tutorial is an example with a simple form and definitions of different types of XSS attacks - reflected XSS, persistent XSS and three ways to prevent them: data filtering, output filtering and data validation. He also links to a few "cheatsheets" to help even more (including this guide and a Zend Framework set of XSS test data.