Super simple form Captcha using RainCaptcha

RainCaptcha is simple to use CAPTCHA for forms, it helps prevent spam submissions by creating images on the fly with embedded text. It can generate an URL to display a CAPTCHA validation image served by the RainCaptcha service.

The class can also send a request to RainCaptcha API to verify if the text that the user entered matches the text in the CAPTCHA image.

RainCaptcha is a CAPTCHA class that does not require any image processing extensions (GD, ImageMagick, etc). CAPTCHA was developed to be readable by humans and resistant to OCR software. It generates black-and-white images with 5 distorted letters on them and noise. Its checking algorithm is case-insensitive.<

Rather then writing a full class to do the heavy lifting make use of the free PHP class already available at  http://raincaptcha.driversworld.us/pages/docs_php_wrapper

To use the class include it on the page then create a new instance, this can then be used to generate a captcha image using ->getImage()

<?php $rainCaptcha = new RainCaptcha(); ?>
<img id="captchaImage" src="<?php echo $rainCaptcha->getImage(); ?>" />
<input name="captcha" type="text" />
<button type="button" class='btn btn-danger' onclick="document.getElementById('captchaImage').src = 
'<?php echo $rainCaptcha->getImage(); ?>&amp;morerandom=' + Math.floor(Math.random() * 10000);"><span class="icon icon-refresh"></span></button>

To check if a user's input matches the captcha

$rainCaptcha = new RainCaptcha();
if(!$rainCaptcha->checkAnswer($_POST['captcha'])){
    die('You have not passed the CAPTCHA test!');
}