<?php
$arr = array(
0 => '3,6',
3 => '4,5',
4 => '7,8',
8 => '9',
);
function writeList($items){
global $arr;
echo '<ul>';
$items = explode(',', $items);
foreach($items as $item){
echo '<li>'.$item;
if(isset($arr[$item]))
writeList($arr[$item]);
echo '</li>';
}
echo '</ul>';
}
writeList($arr[0]);