Prev next posts by menu order

/* REORDER POST NAV */
add_filter( 'get_next_post_where', 'lbardugo_adjacent_post_where', 10, 5 );
add_filter( 'get_previous_post_where', 'lbardugo_adjacent_post_where', 10, 5 );
/**
 * Overrides the post navigation to sort by menu order.
 *
 * @see https://wordpress.stackexchange.com/a/73194/6477
 *
 * @since 1.0.0
 *
 * @param string       $where          The `WHERE` clause in the SQL.
 * @param bool         $in_same_term   Whether post should be in the same taxonomy term.
 * @param int[]|string $excluded_terms Array of excluded term IDs. Empty string if none were provided.
 * @param string       $taxonomy       Taxonomy. Used to identify the term used when `$in_same_term` is true.
 * @param WP_Post      $post           WP_Post object.
 * @return string
 */
function lbardugo_adjacent_post_where($where, $in_same_term, $excluded_terms, $taxonomy, $post ) {

  // Get out now if this isn't the query we want to filter
    if ( !is_main_query() || !is_singular() ) {
        return $where;
    }

    // Check if filtering the next or previous clause.
    // By default, sql is WHERE p.post_date > $date for next and < for previous.
    // To sort by menu order ASC then we need to switch this.
    if (strpos(current_filter(), 'next') !== false) {
        $where = str_replace(">", "<", $where);
    } else {
        $where = str_replace("<", ">", $where);
    }

    $patterns = array();
    $patterns[] = '/post_date/';
    $patterns[] = '/\'[0-9]{4}-[0-9]{2}-[0-9]{2} [0-9]{2}:[0-9]{2}:[0-9]{2}\'/';
    $replacements = array();
    $replacements[] = 'menu_order';
    $replacements[] = $post->menu_order;
    return preg_replace( $patterns, $replacements, $where );

}

add_filter( 'get_next_post_sort', 'lbardugo_adjacent_post_sort', 10, 3 );
add_filter( 'get_previous_post_sort', 'lbardugo_adjacent_post_sort', 10, 3 );
/**
 * Overrides the post navigation to sort by menu order.
 *
 * @see https://github.com/WordPress/wordpress-develop/blob/6.4/src/wp-includes/link-template.php#L1988
 *
 * @since 1.0.0
 *
 * @param string $order_by The `ORDER BY` clause in the SQL.
 * @param WP_Post $post    WP_Post object.
 * @param string  $order   Sort order. 'DESC' for previous post, 'ASC' for next.
 * @return string
 */
function lbardugo_adjacent_post_sort($order_by, $post, $order) {

  // Get out now if this isn't the query we want to filter
    if ( !is_main_query() || !is_singular() ) {
        return $order_by;
    }

    // Check if filtering the next or previous clause.
    if (strpos(current_filter(), 'next') !== false) {
        $order_by = str_replace("ASC", "DESC", $order_by);
    } else {
        $order_by = str_replace("DESC", "ASC", $order_by);
    }

    $pattern = '/post_date/';
    $replacement = 'menu_order';
    return preg_replace( $pattern, $replacement, $order_by );

}