This is not really a captcha script in the typical sense.
A captcha script is easy to obtain but some hosts do not have the gd library which captcha script rely on. Here is a small script I suggested to someone who asked me how to make a captcha without gd.
The script gets two random numbers, adds them together for the answer. The answer is stored in a session variable. The two numbers are displayed for the user and he/she is asked to answer the math problem.
Why did I use sessions and not a <input type = hidden.... because anyone or any bot can simply look at the page source and see the hidden variable and use that as the answer.
PHP Captcha script without gd using text - HttpSeo.com - SEO Tips & Techniques
PHP Captcha script without gd using text
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