Build a tree from a flat array in PHP

build_tree.php

<?php
//--------------------------------------------------------
// Example:
//
//	$cars = [
//		['id' => 999,	'value' =>'CARS',			'parent_id' => 0	],
//		['id' => 11,	'value' =>'Toyota',		'parent_id' => 999],
//		['id' => 1,		'value' =>'Avalon', 	'parent_id' => 11	],
//		['id' => 2,		'value' =>'Corolla', 	'parent_id' => 11	],
//		['id' => 3,		'value' =>'Camry', 		'parent_id' => 11	],
//		['id' => 20,	'value' =>'Skoda',		'parent_id' => 999],
//		['id' => 21,	'value' =>'Octavia',	'parent_id' => 20	],
//		['id' => 22,	'value' =>'Superb',		'parent_id' => 20	],
//		['id' => 211,	'value' =>'Diesel',		'parent_id' => 22	],
//		['id' => 212,	'value' =>'Gasoline',	'parent_id' => 22	],
//	];
//
//	$tree = buildTree($cars);
//
//	echo "<pre>";
//	print_r($tree);
//	echo "<pre>";
//--------------------------------------------------------

function buildTree(array &$elements, $parent_id = 0) {
	$branch = array();

	foreach ($elements as $element) {
		if ($element['parent_id'] == $parent_id) {
			$children = buildTree($elements, $element['id']);
			if ($children) {
				$element['data'] = $children;
			}
			$branch[] = $element;
			unset($elements[$element['id']]);
		}
	}

	return $branch;
}