Display Array as Tree

<?php
$listItems = [
    'Programming' => [
        'PHP' => ['Laravel', 'CodeIgniter'],
        'Python',
        'Java'
    ],
    'Movies' => [
        'The Matrix',
        'Lord of the Rings'
    ],
    'Music',
    'Sports',
    'Livestyle'
];

//printTree
function generateHtmlMenu(array $array)
{
    if (empty($array)) {
        return '';
    }

    $list = '<ul>';
    foreach ($array as $key => $value) {
        $list .= '<li>';
        if (is_array($value)) {
            $list .= $key;
            $list .= generateHtmlMenu($value);
        } else {
            $list .= $value;
        }
        $list .= '</li>';
    }
    $list .= '</ul>';
    return $list;
}

echo generateHtmlMenu($listItems);
?>