SilverStripe 4 (cms4 / SS4): How to make Next / Previous navigation of sibling pages?

I’m looking to create a simple Next and Previous navigation on sibling pages to allow the user to click through them. They are all on a single level. I’ve found some docs and an add-on (link below) but these are aimed at displaying data lists no pages. I cant seem to find any tutorial or information on how this can be achieved. I was advised on the following starting point but not sure how to finish it:

$nextlink = SiteTree::get()->filter(['ParentID' => $this->ParentID, 'Sort:GreaterThan' => $this->Sort])->first()->Link();

https://github.com/fromholdio/silverstripe-paged

https://docs.silverstripe.org/en/4/developer_guides/search/searchcontext/

Answer

uhm, yeah, the code you have there is exactly what you need to get the link of the next page.
Let me break it down:

$nextlink = SiteTree::get()->filter(['ParentID' => $this->ParentID, 'Sort:GreaterThan' => $this->Sort])->first()->Link();

is the one liner version of:

$allPages = SiteTree::get();
$allPagesOnTheSameLevel = $allPages->filter(['ParentID' => $this->ParentID]);
// SilverStripe uses the DB Field "Sort" to decide how to sort Pages. 
// Sort 0 is at the top/beginning, Sort 999... at the end. So if we want the next
// page, we just need to get the first page that has a higher "Sort" value than 
// the current page. Normally ->filter() would search for equal values, but if you 
// add the modifier `:GreaterThan` than it will search with >. And for PreviousPage 
// you can use :LessThan
$currentPageSortValue = $this->Sort;
$allPagesAfterTheCurrentPage = $allPagesOnTheSameLevel->filter(['Sort:GreaterThan' => $currentPageSortValue]);
$nextPageAfterTheCurrentPage = $allPagesAfterTheCurrentPage->first();
if ($nextPageAfterTheCurrentPage && $nextPageAfterTheCurrentPage->exists()) {
  $nextlink = $nextPageAfterTheCurrentPage->Link();
}

this is PHP code, and it assumes that $this is the current page you are viewing.
Assuming you have a standard setup with pages being rendered, you could use it the following way:

(though, I made 1 small modification. Instead of calling ->Link() in php, in the example below I call it in the Template. Instead, I return the full $nextPageAfterTheCurrentPage to the template, this allows me to also use $Title in the template if that is needed)

<?php
// in your app/srv/Page.php

namespace ;

use SilverStripeCMSModelSiteTree;

class Page extends SiteTree {
    // other code here
    
    // add this function:
    public function NextPage() {
        $allPages = SiteTree::get();
        $allPagesAfterTheCurrentPage = $allPages->filter(['ParentID' => $this->ParentID, 'Sort:GreaterThan' => $this->Sort]);
        $nextPageAfterTheCurrentPage = $allPagesAfterTheCurrentPage->first();
        return $nextPageAfterTheCurrentPage;
    }

    // other code here
}

and then, in the template (probably Page.ss), you can do:

<!-- other html here -->
<% if $NextPage %>
    <!-- you can use any propery/method of a page here. $NextPage.ID, $NextPage.MenuTitle, ... -->
    <!-- if you use something inside an html attribute like title="", then add .ATT at the end, this will remove other " characters to avoid invalid html -->
    <a href="$NextPage.Link" title="$NextPage.Title.ATT">To the next Page</a>
<% end_if %>
<!-- other html here -->

---

For previous page, just do the same thing again, but instead of searching/filtering for GraterThan the current Sort, you have to search/filter LessThan.