Primary links theme

I use this function in drupal 4.7 (template.php):

function mytheme_primary_links() {
  $links = menu_primary_links();
  if ($links) {
    $output .= '<ul id="menu">';
    $i = 1;
    foreach ($links as $link) {
      $output .= '<li class="m0'.$i.'">' . $link . '</li>';
      $i++;
    }; 
    $output .= '</ul>';
  }
  return $output;
}

Now, I try the same thing in drupal 6. But the $link variable in foreach statement don't output the link name with href, etc...

The problem is here:

$output .= '<li class="m0'.$i.'">' . $link . '</li>';

Someone know how to solve this issue ? Please help me a bit! Thanks