Exemplo de fazer um menu com submenu

drawmenu.php

<?php 

$pdo = new PDO("mysql:host=localhost;dbname=example", "root", "");

$sql = "SELECT * FROM categories ORDER BY parent, id ASC";
$stmt = $pdo->prepare($sql);
$rows = $stmt->fetchAll();
$results = [];

foreach($row as $row) {
	$results[$row['parent']][$row['id']] = $row;
}

function drawMenu(array $items, $parent=0) {
	
	$menu = '<ul>';
	if(isset($items[$parent])) {
		foreach($items[$parent] as $id => $row) {
			
			$menu .= "<li>";
			$menu .= "<a href='#'>".$row['name']."</a>>";
			if(isset($items[$id])) {
				$menu .= drawMenu($items, $id);
			}
			$menu .= "</li>";
		}
	}
	$menu .= "</ul>";
	return $menu;
}

echo drawMenu($results);