PHP Fatal error: Class 'MY_Controller' not found

I am trying to migrate Codeigniter 2 website from prod to dev and the migration stuck with :
PHP Fatal error: Class 'MY_Controller' not found in /dev/www/application/controllers/theme.php on line 4

The MY_Controller is correctly located in /dev/www/application/core/MY_Controller.php and I am extending the CI_Controller as following :

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

class MY_Controller extends CI_Controller { public $member, $website;

public function __construct() {
parent::__construct();
$this->load->model('common/partner_model');
$this->load->model('common/tracker');
$this->load->helper('partnerid');
$this->initialize_site();
}
…..

Taking in account that file permission, ownership, class and file naming convention are correct and what else should I have a look at?

Ok this issue has nothing to do with Codeigniter. The problem was that the our dev server didn’t allow the PHP's open tag.