PHP - Validating User Submitted Data
http://www.php.net/manual/en/security.variables.php
Essential PHP Security: Code Repository
http://phpsecurity.org/code
Web Application Security Slide by Joe Walker
http://directwebremoting.org/blog/joe/2007/10/29/web_application_security.html
Chris Shiflett: PHP and Web Application Security
http://shiflett.org/