What is CodeIgniter?  

CodeIgniter is a PHP MVC framework used for developing web applications rapidly. CodeIgniter provides out of the box libraries for connecting to the database and performing various operations like sending emails, uploading files, managing sessions, etc.

How CodeIgniter Works?

CodeIgniter is an MVC framework. MVC stands for Model View Controller. When a user requests a resource, the controller responds first. The controller understands the user request then request the necessary data if necessary.

For example, if you want to retrieve a customer with the id= 3, the controller will receive your request, then request the CodeIgniter models to retrieve the record with the id of 3. The CodeIgniter models will return the record to the controller. The controller then forwards the result to the view which formats it into a human-readable format. Then the results are returned to the user in the browser.

The following image shows how CodeIgniter works: