PHP：从数组生成类别/子类别UL列表？

Question

I have an array filled with "Category" objects, each one containing an id and a parent_id value. They're accessed like this:

$category->get("id"); //gets category id
$category->get("parent_id"); gets parent category id

I want to make a HTML UL list like this:

<ul>
    <li>Category</li>
    <li>Category
        <ul>
            <li>Child Category</li>
            <li>Child Category</li>
            <li>Child Category
                <ul>
                    <li>Child Category</li>
                    <li>Child Category</li>
                    <li>Child Category</li>
                </ul>
            </li>
        </ul>
    </li>
    <li>Category</li>
    <li>Category</li>
    <li>Category</li>
    <li>Category</li>
</ul>

What sort of loop must be done to generate this?

Answer 1

The MPTT tree logic might be useful

http://mikehillyer.com/articles/managing-hierarchical-data-in-mysql/