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.

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.

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.

Old stack framework