Yet another captcha implementation in PHP. This one is written with the
current state of captcha-defeating research (as of 2007) in mind. Apart from
a mathematical letter distortion algorithm that is more than just rotating
the letters (just like on the big websites such as Google and Yahoo), the
clutter is designed to make segmentation of the image into separate letter
glyphs hard to do automatically. Finally, the images contain the name of the
originating website to discourage re-using them on porn sites.
If you think this is paranoid, then check out the following references:
- Microsoft research
papers by Kumar Chellapilla: Computer programs are better than humans at
identifying extremely distorted letters.
- OCR Research team's
gallery of weak captchas
- aiCaptcha, a
step-by-step description of how to defeat a distorted-letter captcha.
- Breaking a visual
CAPTCHA: The old captcha at Yahoo defeated.
Anti Spam Scripts 
