menu.php

<?php

class Menu
{
	
	public static function get()
	{
		
		$results = DB::query('SELECT * FROM `menu_items` ORDER BY parent_id IS NULL DESC, parent_id ASC, position ASC')->execute()->as_array();
		
		return static::html(static::children($results));
	}
	
	protected static function children(array $rows)
	{
		$x = array();
		
		foreach($rows as $k => $row)
		{
			// Top Level
			if ($row['parent_id'] == NULL)
			{
				$x[$row['id']] = $row;
				unset($rows[$k]);
			}
			else if (isset($x[$row['parent_id']]))
			{
				$x[$row['parent_id']]['children'][] = $row;
				unset($rows[$k]);
			}
			else
			{
				die('wtf is going on here?!');
			}
		}
		
		return $x;
	}
	
	protected static function html(array $items)
	{
		$html = '';
		
		foreach($items as $item)
		{
			$html .= '<li>' . $item['title'];
			if (isset($item['children']))
			{
				$html .= '<ul>' . static::html($item['children']) . '</ul>';
			}
			$html .= '</li>';
		}
		
		return $html;
	}
}

view.php

<ul><?=Menu::get()?></ul>