Multilevel Menu Codeigniter

//----MY Table--
MariaDB [sik]> select * from sik_institution;
+----+-------------------------------+-----------+--------+
| id | name                          | alias     | parent |
+----+-------------------------------+-----------+--------+
|  1 | Kantor Bendahara Pesantren    | bendahara |   NULL |
|  2 | Bidang Pendidikan Tinggi      | dikti     |      1 |
|  3 | Institut Agama Islam Ibrahimy | iaii      |      2 |
|  4 | Fakultas Syari'ah             | fs        |      3 |
+----+-------------------------------+-----------+--------+
4 rows in set (0.04 sec)



//-----MY_MODEL-------------------------
public function get_menu($id='0')
	{
		$this -> db -> where('id',$id);
		$this -> db -> or_where('parent',$id);
		$res_array = $this -> db -> get('institution') -> result_array();
		return self::html_ordered_menu($res_array,NULL);
	}

function html_ordered_menu($array,$parent_id = 0)
	{
	  $menu_html = '<ul>';
	  foreach($array as $element)
	  {
	    if($element['parent']==$parent_id)
	    {
	      $menu_html .= '<li><a href="'.$element['id'].'">'.$element['name'].'</a>';
	      $menu_html .= self::html_ordered_menu($array,$element['id']);
	      $menu_html .= '</li>';
	    }
	  }
	  $menu_html .= '</ul>';
	  return $menu_html;
	}
	
	
//---MY_CONTROLLER---------------------------
	echo "<pre>";
	
	//works well when i left the parameter blank
		print_r($this -> Institution_model -> get_menu());
	
	//give me nothing
		print_r($this -> Institution_model -> get_menu(2));
	echo "</pre>";
		

Well... if you set get_menu(2) it's normal to receive only those institutions because you simply told the function that you want the elements that start from id 2, namely the elements that either have parent id 2 or id 2.

yeah, but it give me nothing

return self::html_ordered_menu($res_array,$id);

why are you using "self::" and not "$this->"? are you working with static methods?