From Object Oriented perspective, every CakePHP controller is a subclass of the AppController class. Recall how we start the class definition of a controller—look at the following line of code as an example:

class BooksController extends AppController {

Our controllers extend the AppController class—this means we set the AppController class as the parent class of all of our controllers. As of now, it seems alright. But the next question that quickly pops up in our mind is: where is that AppController class located? Well, it can be found inside the app_controller.php file under the /cake/libs/controller/ directory. This app_controller.php is actually a placeholder file and can be overridden by our own one.

The main benefit of having our own AppController class is we can put in common application-wide methods inside this class and all of our controllers will just inherit them. We can use attributes and methods defined inside the AppController from any...