Answer

The code below should be entered within your template.php file and will give the main menu a class of "topnav" and sub-menus a class of "subnav".

function phoenix_menu_tree($tree) {
 return '
'. $tree .'
';
}

function phoenix_menu_item($link, $has_children, $menu = '', $in_active_trail = FALSE, $extra_class = NULL) {
  $class = ($menu ? 'expanded' : ($has_children ? 'collapsed' : 'leaf'));
  if (!empty($extra_class)) {
    $class .= ' '. $extra_class;
  }
  if ($in_active_trail) {
    $class .= ' active-trail';
  }
  $vars['primary_links_tree'] = preg_replace('/
'. $link .$menu."";
}