Tree::flat2nested($arr);

Tree.php

<?php
/*
-----------------------------------
Reformat flat array to nested array
-----------------------------------

Array(
	[2] => Array(
		[id] => 2
		[parent_id] => 0
		[title] => Новости
	)
	[27] => Array(
		[id] => 27
		[parent_id] => 2
		[title] => www-1
	)
	[28] => Array(
		[id] => 28
		[parent_id] => 2
		[title] => www-2
	)
	[3] => Array(
		[id] => 3
		[parent_id] => 0
		[title] => Статьи
	)
	[29] => Array(
		[id] => 29
		[parent_id] => 3
		[title] => www-3
	)
)

Array(
	[2] => Array(
		[id] => 2
		[parent_id] => 0
		[title] => Новости
		[sub] => Array(
			[27] => Array(
				[id] => 27
				[parent_id] => 2
				[title] => www-1
			)
			[28] => Array(
				[id] => 28
				[parent_id] => 2
				[title] => www-2
			)
		)
	)
	[3] => Array(
		[id] => 3
		[parent_id] => 0
		[title] => Статьи
		[sub] => Array(
			[29] => Array(
				[id] => 29
				[parent_id] => 3
				[title] => www-3
			)
		)
	)
)
*/


/**
 * Class Tree
 */
class Tree
{
	/**
	 * Return nested array
	 * @param array $arr_assoc
	 * @return array
	 */
	public static function flat2nested(array $arr_assoc)
	{
		$nested_array = [];
		foreach ($arr_assoc as $id => &$node) {
			if (isset($node['parent_id'])) {
				if ($node['parent_id'] == 0) {
					$nested_array[$id] = &$node;
				} else {
					$arr_assoc[$node['parent_id']]['sub'][$id] = &$node;
				}
			}
		}

		return $nested_array;
	}
}

// Usage
//
Tree::flat2nested($arr);