Page navigation by Next and Prev?

Question

PHP

Now navigation to neighboring pages is based on the parent and neighboring elements. That is, navigation only works relative to the children of the parent element.
PHP code:

<?php
$id = $modx->resource->get('id');
$parentId = $modx->resource->get('parent');

// $children = $modx->getChildIds($parentId,1,array("published"=>1));



$order = "ORDER BY menuindex asc";
$wh=" where published=1 and parent = $parentId ";
$table = 'modx_site_content';
$sql = "SELECT id FROM `$table` ".$wh." ".$order;
$query = new xPDOCriteria($modx, $sql, array());
$children = array();
if ($query->prepare() && $query->stmt->execute()){
    $res = $query->stmt->fetchAll(PDO::FETCH_ASSOC);
    foreach ($res as $row){
        $children[]=$row["id"];
    }
}

// echo "<pre>";
// print_r($children);die;

if (count($children)>0) {
    $key = -1;
    while($key == -1){
     	$key = array_search($id,$children);
    }
    
    $next = (($key+1) == count($children) ? -1 :($key+1));
    $prev = $key-1;
    
    if ($next>=0) {
            $object = $modx->getObject('modResource', $children[$next]);
        	$modx->setPlaceholder('next','<p class="title-next"><a href="/'.$modx->makeUrl($object->get('id')).'">'.$object->get('pagetitle').'</a></p><a href="/'.$modx->makeUrl($object->get('id')).'" class="next"><em></em></a>');
    } else {
            $object = $modx->getObject('modResource', $children[$next]);
          $modx->setPlaceholder('next','<p class="title-next"></p>');
    }
    if ($prev>=0) {
        $object = $modx->getObject('modResource', $children[$prev]);
    	$modx->setPlaceholder('prev','<a href="/'.$modx->makeUrl($object->get('id')).'" class="prev"><em></em></a><p class="title-prev"><a href="/'.$modx->makeUrl($object->get('id')).'">'.$object->get('pagetitle').'</a></p>');
    } else {
        $object = $modx->getObject('modResource', $children[$next]);
        $modx->setPlaceholder('prev','<p class="title-prev"></p>');
    }
}

In such a structure: The
question is whether the same navigation can be implemented only on the "necessary" pages without a parent, for example, by tag, or by id, but the number does not always go in order. Tell me please.

Answer 1

You can slightly correct the query to the database and you can also by tags, or by name ...
As I understand it, how this snippet works, you have no idea?