Rename the file and class to "MY_Controller" if you placed it into the "/core" folder. The prefix is important (which you can set in your config, by the way). Also, make sure, that in MY_Controller, you extend the CI_Controller
class MY_Controller extends CI_Controller
{
public function __construct()
{
parent::__construct();
}
}
You can then access this core-Controller with your other Controllers like this:
class Controller1 extends MY_Controller
{
public function __construct()
{
parent::_construct();
}
}