next prev buttons for posts or pages in wordpress

next_prev.php

<?php
  //next prev buttons
  $pagelist = get_posts('sort_order=asc');//use get_pages for pages
  $pages = array();
  foreach ($pagelist as $page) {
    $pages[] += $page->ID;
  }
  $current = array_search(get_the_ID(), $pages);
  $prevID = $pages[$current-1];
  $nextID = $pages[$current+1];
?>

<a href="<?php echo get_permalink($prevID);?>">Previous</a>
<a href="<?php echo get_permalink($nextID);?>">Next</a>