[eluser]maltzurra[/eluser]
CI_Controller has core Controller functionalities. All Controllers that extend CI_Controller, will have these funcionalities.
For example, in your case:
MY_Controller extends CI_Controller. MY_Controller can do everything CI_Controller does besides the additional testis() method. CI_Controller on the other hand, can't call MY_Controller's own methods.
CI_Controller, can NOT call testis(), as it is MY_Controller's method. This said, if Gallery extends CI_Controller, you will NOT have MY_Controller functionalities and in short, you will not be able to call testis().
If you make Gallery extend MY_Controller, Gallery will have CI_Controller+MY_Controller+own functionalities.
This is called
inheritance.
Hope it helps.