Add "descendants" and "children" property to collection of nodes in nested set format

with_descendants_and_children.php

<?php
/**
 * Adds the children and descendants attribute to an ordered set of nodes
 *
 * @param array $nodes pre-order walk of nodes in nested set format
 */
function with_descendants_and_children(array $nodes) {
  // First we initialise our stack
  $ancestor_stack = [];
  
  // Then we run through ordered nodes
  for ($i = 0; $i < count($nodes); $i++) {
    // Each node has at least an empty descendants/children attribute
    $nodes[$i]['descendants'] = [];
    $nodes[$i]['children'] = [];
      
    // Purge the nodes from our stack that are
    // our sibling sor our ancestors siblings
    while (
      !empty($ancestor_stack) &&
      $nodes[$i]['left'] > $nodes[end($ancestor_stack)]['right']
    ) {
      array_pop($ancestor_stack);
    }
      
    // Each ancestor has this node as a descendant
    foreach ($ancestor_stack as $ancestor_id) {
      $nodes[$ancestor_id]['descendants'][] = $nodes[$i]['title'];
    }
      
    // The last ancestor is also our parent
    $nodes[end($ancestor_stack)]['children'][] = $nodes[$i]['title'];
      
    // In case this node has descendants, we need to add this
    // node to the ancestor stack for future nodes we process
    $ancestor_stack[] = $i;
  }
  
  return $nodes;
}