I have experienced many spam attacks on forms which are used for submission or contact forms. Most spammers use this forms to either mass email or in situations you dont use image verification (captcha) these spams will get into your database. Many hosts are very strict about spams and if they find somebody misusing your mail server for spam your host will suspend your account.
I have collected very useful CAPTCHA scripts are of open source and written in PHP. Most of them are free and are released under GPL license.
Captcha PHP – http://freshmeat.net/p/captchaphp
Quick Captcha – http://www.web1marketing.com/resources/tools/quickcaptcha/
Captcha Generation Class – http://www.ejeliot.com/pages/2
Captcha Script – http://www.coderbari.com/scripts/
Jax Captcha Class – http://www.jtr.de/scripting/php/classes/captcha/index_eng.html
Captcha Creator (commercial) – http://www.captchacreator.com
BotProof Email – It generates image for identification. More info click here
If you want a very professional captchas with varied color and background captcha creator looks promising.
I have heard so many people saying that spammers can easily break captchas by means of optical recognition, until i have seen the real part of it. More information here so that you can understand the other side of its weakness.
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