Retrieves next/prev links for WordPress pages or 'heirarchical' post_types.

next_prev_pages.php

global $post;
$pageID = $post->ID;

$args = array(
  'parent'		=> 0,
	'sort_order'	=> 'ASC',
	'sort_column'	=> 'menu_order',
);
$pagelist = get_pages( $args );

$pages = array();
$x = 0;
foreach( $pagelist as $page ){
	$pages[$x] = $page;
	if( $page->ID == $pageID )
		$current = $x;
	$x++;
}

$prevID = $pages[$current - 1]->ID;
$nextID = $pages[$current + 1]->ID;

echo '<div class="navigation">';
if( !empty( $prevID ) )
	echo '<a href="' . get_permalink( $prevID ) . '" class="btn">Previous: ' . get_the_title( $prevID ) . '</a>';
	
if( !empty( $nextID ) )
	echo '<a href="' . get_permalink( $nextID ) . '" class="btn">Next: ' . get_the_title( $nextID ) . '</a>';
echo '</div>';