custom navigation error — what's wrong here?

<?php
global $post;
  if($post->post_parent) {
$children = wp_list_pages('title_li=&child_of='.$post->ID.'&echo=0');
$menu_title = get_the_title($post->post_parent);
}

if($children) {
	$menu_id = $post->ID;
	$menu_title = get_the_title($post->post_title);
}

else {
	$menu_id = $post->post_parent;
	$parent = get_post($menu_id);
	$menu_title =get_the_title($parent->post_title);
}

 if ($children) { ?>

		<h3 class="secondHeader">
      <a href="?page_id=<?php echo $menu_id; ?>">
      <?php echo $menu_title; ?></a></h3>
		<ul class="secondMenu">
<?php wp_list_pages('title_li=&child_of=' . $menu_id); ?>
  </ul> 

<?php } ?>

when i use this code, nothing displays. Not sure what’s wrong?