CodeIgniter is one of the world’s most popular Open Source PHP frameworks, used by thousands of developers powering hundreds of thousands of sites, in addition to being deployed as the underpinning of every ExpressionEngine installation. As of this writing it is the second most watched PHP project hosted at GitHub, surpassing Slim, Yii, CakePHP, Zend, and Laravel in either followers, contributors, or both. It has the highest number of forks of any PHP project at GitHub of all time. It is used by everyone from AT&T to Home Depot to Dictionary.com, to Rachael Ray to Magento to the Mail & Guardian, to the Universities of Missouri, Michigan, Texas, Georgia, and more (Sources: builtwith.com, wappalyzer.com). And it is used as the server-side back end for many mobile apps.
CodeIgniter is a powerful PHP framework with a very small footprint, built for PHP coders who need a simple and elegant toolkit to create full-featured web applications. CodeIgniter is a proven, agile & open PHP web application framework powering the next generation of web apps.
By Sean Michael Kerner (TechRepublic)
Your clients are a demanding bunch. You, the Web developer, are interested in creating a customized, branded solution to offer to your clients. Customizing a "new" CMS based on an open source CMS framework can bridge the gap between a pure open source CMS product and a commercial CMS.