Next Previous siblings child pages

I tried this piece of code : https://wordpress.stackexchange.com/a/54474/80921 because I couldn’t figure out how to get the next and the previous post. But this code won’t work now and I’m not able to comment.

The error I get is "Undefined offset: -1" when looking for the ID. Here is the full code found on the related page. Thanks for any help !

$  pagelist = get_pages("child_of=".$  post->post_parent."&parent=".$  post->post_parent."&sort_column=menu_order&sort_order=asc"); $  pages = array(); foreach ($  pagelist as $  page) {    $  pages[] += $  page->ID; }  $  current = array_search($  post->ID, $  pages); $  prevID = $  pages[$  current-1]; $  nextID = $  pages[$  current+1]; ?>  <div class="navigation"> <?php if (!empty($  prevID)) { ?> <div class="previous"> <a href="<?php echo get_permalink($  prevID); ?>" title="<?php echo get_the_title($  prevID); ?>">Previous</a>  </div> <?php } if (!empty($  nextID)) { ?> <div class="Next"> <a href="<?php echo get_permalink($  nextID); ?>" title="<?php echo get_the_title($  nextID); ?>">Next</a> </div> <?php } ?> </div>