Original text : 
  
  <?php if (count($primary_links)) : ?>
    <ul id="primary">
    <?php foreach ($primary_links as $link): ?>
      <li><?php print $link?></li>
    <?php endforeach; ?>
    </ul>
  <?php endif; ?>

Turns into :
  
  <?php if (count($primary_links)) : ?>
    <ul id="primary">
    <?php foreach ($primary_links as $path => $link): ?>
      <li <?php print (drupal_get_normal_path($path) == $_GET['q']) ? 'class="active"' : ''   ?>><?php print $link?></li>
    <?php endforeach; ?>
    </ul>
  <?php endif; ?>