Download this package and uncompress it within application/ directory of your CodeIgniter site.
Then activate Session library/driver. GD image processing PHP module should be installed.
This feature has been tested on CodeIgniter 3.1.11 (September 19th, 2019).
Open with browser the following URL:
http://localhost/site/index.php/captcha_test
or
http://localhost/site/captcha_test (if you removed the URL-segment index.php).
Examine code of the controller Captcha_test.php and the views captcha_test.php and captcha_test_ok.php. This would be enough for you to get the idea how captcha feature works.
Copyright by Kruglov Sergei, 2006, 2007, 2008, 2011
http://www.captcha.ru
http://www.kruglov.ru
KCAPTCHA is a free software. You can freely use it for developing own site or software. If you use this software as a part of own sofware, you must leave copyright notices intact or add KCAPTCHA copyright notices to own. As a default configuration, KCAPTCHA has a small credits text at bottom of CAPTCHA image. You can remove it, but I would be pleased if you left it. ;)
Ported for CodeIgniter by Ivan Tcholakov, 2013-2022.
The demonstration code is under the MIT License, http://opensource.org/licenses/MIT