extending the controller

I am attempting to follow CodeIgniter tutorials that focus on extending the controller. The tutorial advises placing the following code in the MY_Controller file:

PHP Code:

<?php if (!defined('BASEPATH')) exit('No direct script access allowed');

class MY_Controller extends Controller
{
    function __construct()
    {
        parent::__construct();
    }
} 


However, this results in the following error:
Fatal error: Class 'Controller' not found in /home/chimeri1/public_html/application/libraries/MY_Controller.php on line 3
If I substitute CI_Controller instead of Controller, the code works. What is the difference between the two?
Also, should the closing PHP tag ?> be included at the end of the MY_Controller file? Some tutorials omit it.
Thank you in advance for your assistance.

It's CI_Controller not Controller for CodeIgniter 3.
CodeIgniter 4 uses Controller.

PHP Code:

<?php if (!defined('BASEPATH')) exit('No direct script access allowed');

class MY_Controller extends CI_Controller
{
    function __construct()
    {
        parent::__construct();
    }
} 


What did you Try? What did you Get? What did you Expect?

Joined CodeIgniter Community 2009.  ( Skype: insitfx )