Strict Standards: Declaration of description_walker::start_el() and warning [duplicate]

Not sure you’re asking for error with Walker_Nav_Menu::start_el() or Walker_Nav_Menu::start_lvl(). But you must specify all arguments as default in the Walker_Nav_Menu class in your derived classes. E.g:

class description_walker extends Walker_Nav_Menu
{
  /**
   * Starts the list before the elements are added.
   *
   * @see Walker::start_lvl()
   */
  public function start_lvl(&$output, $depth = 0, $args = array())
  {
    // your code...
  }

  /**
   * Start the element output.
   *
   * @see Walker::start_el()
   */
  public function start_el(&$output, $item, $depth = 0, $args = array(), $id = 0)
  {
    // your code...
  }
}