On the NETTUTS.com website, there's a great article with some "essential security considerations" that you can use to see just how hackable your site could be.

This article walks through the brainstorming stage of planning for what is in this instance, a hypothetical user-centric web application. Although you won’t be left with a complete project – nor a market ready framework, my hope is that each of you, when faced with future workloads, may muse on the better practices described. So, without further ado...Are you sitting comfortably?

The tutorial is broken up into a few sections based around an example with a few points of failure (about book information). They work through the thought process behind the code, using the $_REQUEST variables correctly, preventing SQL injections, filtering the HTML output and a sample code download for you to see how it's all tied together.