WordPress sidebar example. Conditional loading of a dynamic sidebar or a link, based on a custom variable.

condional-sidebar.php

<div id="sidebar">

<?php
// Check for the 'part' variable
$part = get_query_var('part');

// If this is a sidebar, show the dynamic content
if ( $part == 'sidebar' ) :
?>

  <div id="sidebar-content">
  <?php 
    dynamic_sidebar();
  ?>
  </div>

<?php
// Else, just show a link pointing to the desired endpoint
else : ?>
  <a href="part/sidebar/" id="load-asides">Related content</a>
<?php endif; ?>
</div>