WP : next / prev post link

extras.php

<?php
/**
* Return the right previous_post_link / next_post_link when change posts order
* ref: https://1fix.io/blog/2014/09/09/get-right-previous_post_link-when-order-posts-by-menu_order/
* more reading : http://wordpress.stackexchange.com/questions/73190/can-the-next-prev-post-links-be-ordered-by-menu-order-or-by-a-meta-key
*/

function kms_previous_post_where() {
 	global $post, $wpdb;
 	return $wpdb->prepare( "WHERE p.menu_order < %s AND p.post_type = %s AND p.post_status = 'publish'", $post->menu_order, $post->post_type);
 }
 add_filter( 'get_previous_post_where', __NAMESPACE__.'\\kms_previous_post_where' );

 function kms_next_post_where() {
 	global $post, $wpdb;
 	return $wpdb->prepare( "WHERE p.menu_order > %s AND p.post_type = %s AND p.post_status = 'publish'", $post->menu_order, $post->post_type);
 }
 add_filter( 'get_next_post_where', __NAMESPACE__.'\\kms_next_post_where' );

 function kms_previous_post_sort() {
 	return "ORDER BY p.menu_order desc LIMIT 1";
 }
 add_filter( 'get_previous_post_sort', __NAMESPACE__.'\\kms_previous_post_sort' );

 function kms_next_post_sort() {
 	return "ORDER BY p.menu_order asc LIMIT 1";
 }
 add_filter( 'get_next_post_sort', __NAMESPACE__.'\\kms_next_post_sort' );