<?php $data = array(
    3 => array(
        4 => array(
            7 => null,
            8 => array(
                9 => null
            )
        ),
        5 => null
    ),
    6 => null
);

function writeList($tree)
{
    if($tree == NULL) return;
    echo "<ul>";
    foreach($tree as $node=>$children)
        echo "<li>", $node, writeList($children), '</li>';
    echo "</ul>";
}

writeList($data);

<ul><li>3<ul><li>4<ul><li>7</li><li>8<ul><li>9</li></ul></li></ul></li><li>5</li></ul></li><li>6</li></ul>