None of the other answers in here actually solved this problem for me.

In this particular case I was using CodeIgniter and adding any of the following lines before the line that caused the error:

 $this->load->model('Authnet_Class');

OR

 get_instance()->load->model('Authnet_Class')

OR

 include APPPATH . '/model/Authnet_Class.php';

Did not solve the problem.

I managed to solve it by invoking the class definition in the construct of the class where I was accessing Authnet_Class. I.e.:

class MY_Current_Context_Class extends CI_Controller {

    public function __construct() {
        parent::__construct();
        $this->load->model('Authnet_Class');
    }
    // somewhere below in another function I access Authnet_Class ...

I now understand that the context where you access the Authnet_Class class, needs to have its definition present on the context’s class construct (and not just before you invoke the properties of Authnet_Class).