Adjacency Model Ik krijg het niet voor elkaar

CREATE TABLE IF NOT EXISTS `navigatie` (
  `id` tinyint(4) NOT NULL AUTO_INCREMENT,
  `parent` tinyint(4) NOT NULL DEFAULT '0',
  `url` varchar(64) NOT NULL,
  `label` varchar(64) NOT NULL,
  `sequence` tinyint(4) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `navigatie` (`id`, `parent`, `url`, `label`, `sequence`) VALUES
(1, 0, '/', 'Home', 1),
(2, 0, 'javascript:void(0)', 'Diensten', 2),
(3, 0, 'avascript:void(0)', 'Tarieven', 3),
(4, 0, '/case_studies', 'Case Strudies', 4),
(5, 0, '/over', 'Over', 5),
(6, 2, '/diensten/online_strategie', 'Online Strategie', 1),
(7, 2, '/diensten/webdesign', 'Web Design', 2),
(8, 2, '/diensten/webdevelopment', 'Web Development', 3),
(9, 2, '/diensten/digital_marketing', 'Digital Marketing', 4);

	public function get_menu()
	{
		$sql = $this->pdo->query('SELECT * FROM site_navigation ORDER BY parent, sequence ASC');
		
		$items	= array();
		$rows = $sql->fetchAll(PDO::FETCH_ASSOC);
		foreach ($rows as $r => $row) {
			$parent = (int)$row['parent'];
			if (!isset($items[$parent])) {
				$items[$parent][] = $row;
			}
		}
		foreach ($items[0] as $m => $item) {
			echo '<ul class="navbar-nav ml-auto">';
				echo '<li>'.$item['label'].'</li>';
				$this->get_sub_menu($item['id']);
			echo '</ul>';
		}
		return $items;
	}