I used CodeIgniter while working at KHM Travel Group. It was the system we used for an affiliate-based site called TravSearch, which I later replaced with a WordPress Multisite. I developed a .csv upload form to transfer all the information from the affiliate table of CodeIgniter to the user and options tables in WPMU.

CakePHP is used because it is a very feature-complete, battle-tested rapid development framework. This saves us months of development time because the API's do most of the work. There is also an extensive plugin and community built around this platform.

CodeIgniter is the MVC framework used for the PHP coding. Also, validation criteria for (database) table fields are the provided by CodeIgniter, and signatures for others are derived from those.

CakePHP is one of the most mature and developed PHP Frameworks available, it brings order to Chaos and the core team are a decent bunch of people who really care about the project

It is super easy to do things in PHP using codeigniter. Used the MVC architecture to create different types of pages and modules.

Integrate with some code-igniter low-level modules through composer.

Admin interface for users.

Old stack framework