PHP: PHP GD Library: Captcha and the Alternatives

Most of us have probably encountered a Captcha system on the web in the past few years, sometimes without even knowing it. Captcha systems are designed to filter out automated "spam bots" from posting to websites that accept submissions such as forums and blogs by asking the user to solve a problem. These problems can range from typing in the letters and numbers displayed in an image to answering a simple math question or selecting the correct image.