If i’ve got 2(or more) model methods which do (for example, in billing system) enrolling/withdrawing, and one controller’s method that calls 2(or more) of these model methods.

Is it a good way(maybe, any suggestions how to do it better) to write/use 2model methods like these:

public function start_transaction(){
    $this->db->trans_start();
}

public function end_transaction(){
   $this->db->trans_complete();
} 

And call in controller’s method:

   public function smth(){
       //something
       $this->model->start_transaction();
       $this->model->enroll();
       //something else
       $this->model->withdraw();
       $this->model->end_transaction();
   } 

Will transaction be reversed, if model's withdraw() method fails?

Thanks.