A CAPTCHA is an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart", it is a type of challenge-response test used in computing to determine whether or not the user is human. This test can only be solved by a human, thus, any correct answer for this test is human's answer not a bot.
ArCaptcha (Arabic Captcha) is an open-source free PHP CAPTCHA script for generating complex images and CAPTCHA codes to protect forms from spam and abuse. It can be easily added into existing forms on your website to provide protection from spam bots. It can run on most any webserver as long as you have PHP installed, and GD support within PHP. ArCaptcha does everything from generating the CAPTCHA images to validating the typed code.