php 28 lines · 6 steps

Building a tree from a flat list in PHP

Group rows by parent once, then recurse to nest each node's children.

Explained by highlit
Intermediate recursion tree grouping adjacency-list data-structures
1<?php
2 
3function buildTree(array $items, ?int $parentId = null): array
4{
5 $indexed = [];
6 foreach ($items as $item) {
7 $indexed[$item['parent_id'] ?? 0][] = $item;
8 }
9 
10 return attachChildren($indexed, $parentId ?? 0);
11}
12 
13function attachChildren(array $indexed, int $parentId): array
14{
15 $branch = [];
16 
17 foreach ($indexed[$parentId] ?? [] as $item) {
18 $children = attachChildren($indexed, $item['id']);
19 
20 if ($children !== []) {
21 $item['children'] = $children;
22 }
23 
24 $branch[] = $item;
25 }
26 
27 return $branch;
28}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Indexing rows by parent once turns repeated O(n) scans into O(1) lookups per node.
  2. 2Recursion mirrors the tree's shape: each call handles one level and delegates the rest.
  3. 3Only attaching a children key when children exist keeps leaf nodes clean.

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