php tree ul li hierarchy menu from array

Question

we have this array from mysqli query output :

$items = Array
(
    Array
    (
        'id' => 1,
        'title' => 'menu1',
        'parent_id' => 0
    ),
    Array
    (
        'id' => 2,
        'title' => 'submenu1-1',
        'parent_id' => 1
    ),
    Array
    (
        'id' => 3,
        'title' => 'submenu1-2',
        'parent_id' => 1
    ),
    Array
    (
        'id' => 4,
        'title' => 'menu2',
        'parent_id' => 0
    ),
    Array
    (
        'id' => 5,
        'title' => 'submenu2-1',
        'parent_id' => 4
    )
);

and we need this html output with php :

<ul>
        <li><a>menu1</a>
             <ul>
                 <li><a>submenu1-1</a></li>
                 <li><a>submenu1-2</a></li>
             </ul>
        </li>
        <li><a>menu2</a>
             <ul>
                 <li><a>submenu2-1</a></li>
             </ul>
        </li>
</ul>

can anyone help me ? Probably this is very easy but I have tried everything already without success !!

Answer 1

//index elements by id
foreach ($items as $item) {
    $item['subs'] = array();
    $indexedItems[$item['id']] = (object) $item;
}


//assign to parent
$topLevel = array();
foreach ($indexedItems as $item) {
    if ($item->parent_id == 0) {
        $topLevel[] = $item;
    } else {
        $indexedItems[$item->parent_id]->subs[] = $item;
    }
}

//recursive function
function renderMenu($items) {
    $render = '<ul>';

    foreach ($items as $item) {
        $render .= '<li>' . $item->title;
        if (!empty($item->subs)) {
            $render .= renderMenu($item->subs);
        }
        $render .= '</li>';
    }

    return $render . '</ul>';
}

echo renderMenu($topLevel);