reCAPTCHA helps prevent automated abuse of your site (such as comment spam or bogus registrations) by using a CAPTCHA to ensure that only humans perform certain actions.

• It’s Free! Yep, reCAPTCHA is free. The only reason we might charge is if you require special services from us.
• It’s Useful. Why waste the effort of your users? By using reCAPTCHA instead of other CAPTCHA implementations, you are helping to digitize books.
• It’s Easy. reCAPTCHA is a Web service. As such, adopting it is as simple as adding 4 lines of code on your site. For many applications and programming languages such as WordPress and PHP we also have easy-to-install plugins available. We generate and check the distorted images, so you don’t need to run costly image generation programs.
• It’s Accessible. Most other implementations of CAPTCHAs block visually impaired individuals, who cannot read images of distorted text. reCAPTCHA, on the other hand, has an audio test that allows blind people to freely navigate your site.
• It’s Secure. reCAPTCHA is run by the original creators of CAPTCHA and has the highest security standards. Many other implementations of CAPTCHAs can be easily broken.

Visual and Audio PHP CAPTCHA Generation Class

PhpCaptcha is a library for generating visual and audio CAPTCHAs (completely automated public Turing test to tell computers and humans apart). You can read more about CAPTCHAs at Wikipedia.

It can help you to prevent/reduce:

• Automated sign-ups in registration forms.
• Comment spam in blogs and guestbooks.
• Brute force attacks on login systems.

KCAPTCHA is a free and open source PHP solution to generate human validation images (CAPTCHA).

KCAPTCHA is meant to be a very strong protected one but requires no special hosting features, only PHP with GD library.

How it works: script creates session and writes to it with name $_SESSION['captcha_keystring'] randomly generated text string. And outputs image with distorted text. While checking visitor humanity, you should compare code string from session with one from visitor input. Be sure that session contains non-empty code string!

System requirements: PHP v. 4.0.6 or above with GD2 library support. Not required font libraries like Libttf etc.

The package contains pixel fonts, so you should not search and install fonts.

You can customize colors, character sets, string and image size.

identiPIC is a photo CAPTCHA system whereby a website visitor has to identify a picture to proceed. The system is typically used on web forms to block spambots. A human can identify a picture, a spambot cannot.

The web is awash with spambots that attack contact forms, guestbooks, forums etc. Up until now, the main line of defence against them was the CAPTCHA. A CAPTCHA is a test to tell humans apart from computers (spambots in this case) that relies on the user entering a string of characters presented to them in an image. This worked well for a while until the bots began to overcome them by reading them with OCR (optical character recognition) programs. Another problem with the traditional CAPTCHA is that they are sometimes difficult to read. The advantage of the identiPIC system is that a computer cannot tell what a picture is of but a human can clearly see what it is.

CAPTCHA is a simple test to determine if a user is a computer or a human. It is used to prevent spam abuse on the websites. So if you use CAPTCHA on your web site forms, this can help in stopping some bots and making life harder for other bots in accessing or using your forms.

In brief the CAPTCHA protection works by generating a random string, writing it to an image, then storing the string inside of a session or by some other method. This is then checked when the form is submitted.

The goal of this tutorial is to demonstrate how to make your own simple CAPTCHA protection using PHP and AJAX technologies.

Creating this website made with php, I attracted lots of spambots to flood my forum, my programming resources, freelance database and other forms, but you probably know this situation very well.  So I decided to create a simple anti-spam php Class which could help me and probably other people to deal with the spam.

MathGuard is free php class that everyone can use as anti-spam protection for the website forms. It’s also a suitable solution when there is no php GD support from the webhosting company.