Expanded menu question

Hey I have expanded menu like this: (http://drupal.org/node/31307)

<?php 
if ($menu_tree = my_menu_tree(63)) print "<ul id='primary-nav'>".$menu_tree."</ul>";

function my_menu_tree($pid = 0) { 
  $menu = menu_get_menu($pid); 
  $output = ''; 

  foreach ($menu['visible'][$pid]['children'] as $mid) {
    if (count($menu['visible'][$mid]['children']) > 0) {
      $output.= "<li class='menuparent'>".menu_item_link($mid); //."</li>";
      $output.= "<ul>";
      $output.= my_menu_tree($mid);
      $output.= "</ul>";
      $output.= "</li>";
    } 
    else {
      $output.= "<li>".menu_item_link($mid)."</li>";
    }
  }
  return $output;
}
?>

I want to replace class='menuparent' to class='selected', of course only when user is on this page or it's sub-pages.

sorry for my bad english
Bartek