Problem with callback & validation

Code:
&lt;?php



class Registrieren extends Controller {

    

    function Registrieren()

    {

        parent::Controller();    

    }

    

    function index()

    {        

        $data['titel'] = 'Kostenlos Registrieren';

        

        // Formular abgeschickt ?

        if(isset($_POST['submit-registrieren'])) {            

            $this->load->library('validation');

            $this->validation->set_error_delimiters('', '<br>');

    

            $rules['username'] = "callback__userCheck";

            $rules['password'] = "trim|required|min_length[3]|max_length[20]|xss_clean";

            $rules['email'] = "trim|required|valid_email|xss_clean";

            

            $this->validation->set_rules($rules);

            

            $fields['username'] = 'Username';

            $fields['password'] = 'Passwort';

            $fields['email'] = 'e-Mail';

            

            $this->validation->set_fields($fields);

        

            if ($this->validation->run() == TRUE){

                //$this->load->view('registrieren/erfolgreich_view');

                //die();

            }



            function _userCheck($str) {



                $query = $this->db->query("SELECT id,username FROM user WHERE username = ".$this->db->escape($str)." LIMIT 1;");

                

                if($query->num_rows() > 0) {

                    $this->validation->set_message('_userCheck', 'Dieser Username ist schon in der Datenbank');

                    return FALSE;

                }

                else {

                    return TRUE;

                }

            }

            

        }

        

        

        $this->load->view('registrieren/index_view', $data);

    }

    

    

}