Converts flat array into tree, avoiding the necromancy with references in PHP

array2tree_converter.php

<?php

class Array2Tree
{
	public static function convert(array $data)
	{
		$instance = new self($data);
		return $instance->getTree();
	}
	
	public function getTree()
	{
		$this->makeObjects();
		$this->buildObjectTree();
		return $this->toArray($this->root->child);
	}
	
	private function __construct(array $data)
	{
		$this->data = $data;
		$this->objs = [];
		$this->root = new stdClass();
	}
	
	private function makeObjects()
	{
		foreach ($this->data as $item) {
			$this->objs[$item['id']] = new stdClass();
			
      		// Define your own fields here:
			$this->objs[$item['id']]->id = $item['id'];
			$this->objs[$item['id']]->parent = $item['parent'];
			$this->objs[$item['id']]->name = $item['name'];
      		// ...
		}
	}
	
	private function buildObjectTree()
	{
		foreach ($this->objs as &$item) {
			if ($item->parent && isset($this->objs[$item->parent])) {
				$this->objs[$item->parent]->child[$item->id] = $item;
			} else {
				$this->root->child[$item->id] = $item;
			}
		}
	}
	
	private function toArray($data)
	{
		if (is_object($data)) {
			$data = get_object_vars($data);
		}
		
		if (is_array($data)) {
			$data = array_map([$this, __FUNCTION__], $data);
		}
		
		return $data;
	}
}

usage.php

<?php

$data = [
  		[
			'id' => '1',
			'parent' => NULL,
			'name' => 'This will be in root',
		],
		[
			'id' => '2',
			'parent' => '1',
			'name' => 'This will be nested',
		],
		[
			'id' => '3',
			'parent' => '999',
			'name' => 'This will be in root too, because of invalid parent',
		],
];

$tree = Array2Tree::convert($data);
print_r($tree);