GD Image Verification Class: Generate CAPTCHA verification images for forms

This class can be used to perform CAPTCHA tests to verify whether a site user is a real human or a robot script.

It generates images with random secret texts. The secret texts are stored on the server side using session variables.

The images are drawn with the GD library and outputted in the JPEG format.

The class also provides a verification function to determine whether the text in the image matches the text eventually entered by an user in a verification form.