Zend Framework has a set of great classes included to take away a lot of the headaches involved with setting up a CAPTCHA system. The one I use is Zend_Captcha_Image, which creates an image for you and sets up a Session so you will be able to validate all information appropriately. Problem is, there's little documentation out there on this. [...] I'll show you everything you need to know to use Zend_Captcha_Image WITHOUT Zend_Form! If you'd like to learn the implementation using Zend_Form, please visit Robert Basic's Blog.

He follows the steps you'll need to get the component set up - checking for GD installation, making the CAPTCHA image folder - and includes some sample code in the form of a generateCaptcha and validateCaptcha methods to make it all work.

I'll just show a quick example how to implement Zend_Captcha into a Zend_Form, may be useful for someone. There are several CAPTCHA types in ZF, like the Image, Figlet and Dumb. I use Image.

He includes the code for his form where the CAPTCHA component is configured to use a certain directories for the location to put the images, the font to use, and the label to put on it. He sets up the action for it and creates a captchaAction method in the same controller. This does the validation on the entered term to ensure that it matches the string in the image exactly.