Clarification about MY_Controller and relative inheritances

It will do the job, but I don't think it's the best practice.

Once again I do not understand the above statement. The pronoun's antecedent is vague in the extreme. To put that more clearly, what the heck is the "it" you're talking about?

Or simply use the framework's convention to autoload your files.

As I pointed out, the framework's "convention to autoload" design is not always adequate. For instance, consider two custom libraries.

File: application/libraries/CustomLib1.php

PHP Code:

class CustomLib1
{
    ...
} 


And a second class that extends CustomLib1

File: application/libraries/CustomLib2.php

PHP Code:

class CustomLib2 extends CustomLib1
{
 public function __construct()
 {
        parent::__construct();
 }
} 


If you're relying on the framework's "convention to autoload" and make the following call

PHP Code:

$this->load->library('CustomLib2'); 


You will receive a fatal error stating that Class 'CustomLib1' cannot be found.
To overcome this problem you have a couple options.

First option: Manually include the base class file before trying to load the child class.

PHP Code:

include APPPATH . 'libraries/CustomLib1.php';
$this->load->library('CustomLib2'); 


Second option: Implement an autoloader.

The second option is what I prefer.

I'm happy to hear that you know spl_autoload_register is used to implement an autoloader.