PHP implementation of security text

If you're looking for a way to prevent brute-force attacks against your login pages, automated sign-ups through your registration forms, or automated spam in your blog commenting system, then look no further! In this article, I'll guide you through the basics of creating and integrating a security image like those found on the sign-up pages of many mainstream sites.

http://www.sitepoint.com/article/toughen-forms-security-image/