Using a class as property in another class

I am using CI for years. I only have controllers, so no views or model classes.
I migrated to the latest version 3.1.5 by unzipping the CI zip file and only copying my \application\*.php files.  I read all migration stuf and googled on it.

In my file Ezbuild_ezm_tools.php I got this issue:

When I am running this code working fine on CI 2.

PHP Code:

$this->mm = new Ezbuild_ezm_mm (); 


I got this message on CI. Unable to locate the specified class: Session.php

Ezbuild_ezm_mm is also an other extension of CI_controller.

When I remove this previous line no error message. So the cause of the error is determined.

So I tried/changed the line to this:

PHP Code:

    $this->load->library('ezbuild_ezm_mm');
    $this->ezbuild_ezm_mm->say_hello(); 


This generates a new issue:
Undefined property: Ezbuild_ezm_tools::$ezbuild_ezm_mm on this line

PHP Code:

$this->ezbuild_ezm_mm->say_hello(); 


My classes are renamed according the 3 way like this:

PHP Code:

class Ezbuild extends CI_Controller {

  function __construct() {
    parent::__construct(); 


After this i am extending all my classes like this:

PHP Code:

require_once("Ezbuild.php");

class Ezbuild_ezm_main_class extends Ezbuild { 


Any help appreciated! Thanks in advance.

John