Help rewrinting primary links classes

Hi guys, I need help in understanding class rewrinting of drupal function theme_links.

I have found lot of solutions on the web but i'm new in php and i cant understand how to do some things.

Basically I need the primary links, the ul id of ma primary links must be nav and the class sf-menu

I have accomplished with this snippet and it works like a charm.

  print theme('links', $primary_links, array('id' => 'nav','class' => 'sf-menu'))

My ul has id = nav and class = sf-menu

Now my main problem is one:

i need every tag li with the class -> first-level-li
and every tag a (who belong to the li tag) with the class first-level-link

Now i have this situation:

<li class="menu-504 active-trail first active">
      <a href="/" title="" class="active">Home</a>
</li>

What i'm trying to accomplish is:

<li class="first-level-li">
      <a href="/" title="" class="first-level-link active">Home</a>
</li>

I have found this snippet but doesn't works well, for example, this part:

// Automatically add a class to each link and also to each LI

doesn't work for me and i have no dk_three on items.

 function yourThemeName_links($links, $attributes = array('class' => 'links')) {
  $output = '';

  if (count($links) > 0) {
    $output = '<ul' . drupal_attributes($attributes) . '>';

    $num_links = count($links);
    $i = 1;

    foreach ($links as $key => $link) {
      $class = $key;

      // Automatically add a class to each link and also to each LI
      if (isset($link['attributes']) && isset($link['attributes']['class'])) {
        $link['attributes']['class'] .= 'dk_three ' . $key;
      }
      else {
        $link['attributes']['class'] = $key;
      }

      // Add first and last classes to the list of links to help out themers.
      $extra_class = '';
      if ($i == 1) {
        $extra_class .= 'first dk_four ';
      }
      if ($i == $num_links) {
        $extra_class .= 'last dk_five ';
      }
      $output .= '<li ' . drupal_attributes(array('class' => $extra_class . $class)) . '>';

      // Is the title HTML?
      $html = isset($link['html']) && $link['html'];

      // Initialize fragment and query variables.
      $link['query'] = isset($link['query']) ? $link['query'] : NULL;
      $link['fragment'] = isset($link['fragment']) ? $link['fragment'] : NULL;

      if (isset($link['href'])) {
        $output .= l($link['title'], $link['href'], $link['attributes'], $link['query'], $link['fragment'], FALSE, $html);
      }
      else if ($link['title']) {
        //Some links are actually not links, but we wrap these in <span> for adding title and class attributes
        if (!$html) {
          $link['title'] = check_plain($link['title']);
        }
        $output .= '<span' . drupal_attributes($link['attributes']) . '>' . $link['title'] . '</span>';
      }

      $i++;
      $output .= "</li>\n";
    }

    $output .= '</ul>';
  }

  return $output;
}

Someone can please help me to make this function? Making examples of code?

Thx a lot
Luke