Boutell's Simple PHP Captcha.
	
Copyright 2007, Thomas Boutell and Boutell.Com, Inc. 

Permission granted to use and modify this code as you see fit, provided 
that you do not represent this code as your own original work when
redistributing it to others for use on their own sites.

Otherwise, feel free to profit from this code in any way you wish.

I ask, but do not require, that you link to the original article 
if you find this software useful:

http://www.boutell.com/newfaq/creating/captcha.html

See the above article for complete details on requirements to
correctly use this software.