Mutlilevel page list in CodeIgniter

I'm newly working with CodeIgniter and PHP. I wrote a category structured 'pages' code with listing pages as multilevel list or indented select box.

page                                        page_lang
-------------------------------------       ----------------------------
id_page | id_menu | id_parent | level       id_page |  title     | etc..
-------------------------------------       ----------------------------
   1    |    1    |     0     |   0            1    | Name 1     | etc..
   2    |    1    |     1     |   1            2    | Name 1.1   | etc..
   3    |    1    |     2     |   2            3    | Name 1.1.1 | etc..
   4    |    1    |     2     |   1            4    | Name 1.2   | etc.

My controller:

function get_menu_content($id_menu, $id_parent = 0, $level = 0)
{
  echo '<select>';
  $dropdown = $this->my_model->get_menu($id_menu,'dropdown');
  echo $dropdown;
  echo '</select>';  //echo in controller for quick test purpose
}

My_model has three functions: the first is getting result array, the other two is rendering as list and options.

class My_model extends CI_Model
{

  public $model_result;

  function __construct()
  {
     parent::__construct();
  }

function get_menu($id_menu = '1',$type = 'list')
{
  $sql = "
  SELECT a.id_parent, b.id_page, a.level, b.title 
  FROM page a
  INNER JOIN page_lang b ON a.id_page = b.id_page 
  WHERE 
    a.id_menu = $id_menu";  
  $query = $this->db->query($sql);  

  if($query->num_rows() > 0)
  {
    $res = $query->result_array();
    foreach($res as $row):
      $items[$row['id_parent']][] = $row;
    endforeach;
    if($type =='list') $this->_menu_list($items); //renders ul tags
    if($type =='dropdown') $this->_menu_dropdown($items); //renders option tag
    return $this->model_result;
  }
}

function _menu_list($items, $parent = null)
{
  $index = $parent == null ? '0' : $parent;  
  if (isset($items[$index]))
  {
    $this->model_result .= '<ul';
    $this->model_result .= $parent == null ? ' id="category" ' : '';
    $this->model_result .= '>';    
    foreach ($items[$index] as $child)
    {
      $this->model_result .= '<li rel="'.$child['id_page'].'">'.$child['title'];
      $this->_menu_list($items, $child['id_page']);
      $this->model_result .= '</li>';
    }    
    $this->model_result .= '</ul>';
  }
}

function _menu_dropdown($items, $parent = null, $level = '0')
{
  $index = $parent == null ? '0' : $parent;
  $space = str_repeat(' &nbsp &nbsp ', $level);
  if (isset($items[$index]))
  { 
    foreach ($items[$index] as $child)
    { 
      $this->model_result .= '<option value="'.$child['id_page'].'">'.$space.$child['title'].'</option>' . "\n";
      $this->_menu_dropdown($items, $child['id_page'],$level+1);
    }
  }
}

It works a bit slowly. I need a cleaner code I guess. I will appreciate any improvement on the code based on my database structure.