On PHPBuilder.com today, there's a new tutorial that looks at a method to protect your PHP applications with the filters the Open Web Application Security Project provides.
OWASP (Open Web Application Security Project ) released a top ten list for web application security vulnerabilities in 2003 and 2004; you can find the latest information about their Top Ten Project here.
Most of the top ten vulnerabilities including (A1) Unvalidated Input, (A2) Broken Access Control, (A4) Cross Site Scripting (XSS) Flaws, and (A6) Injection Flaws, can be avoided by using these filters.
They walk through the installation before talking about the types of filters at your disposal - paranoid, SQL, system, HTML, int/float, UTF-8, and LDAP. The check() function looks at the inputted value to see if it passes the test (the other option the function takes).
They also give an example of combining filters as well, making more secure validation even easier.