Convert this array to an HTML List

I have this array:

array
  0 => string '3,6' (length=3)
  3 => string '4,5' (length=3)
  4 => string '7,8' (length=3)
  8 => string '9' (length=1)

OR

array
  3 => 
    array
      4 => 
        array
          7 => null
          8 => 
            array
              9 => null
      5 => null
  6 => null

Every key is the id and value is the id for the childs of this parent.
ID of 0 means that (3 & 6) have no parent.
Now, i want to output an HTML list like:

• 3
  • 4
    • 7
    • 8
      • 9
  • 5
• 6

Answer

$arr = array(
  0 => '3,6',
  3 => '4,5',
  4 => '7,8',
  8 => '9',
);
function writeList($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]);

Test it.

or

$arr = array(
    3 => array(
        4 => array(
            7 => null,
            8 => array(
                9 => null
            ),
        ),
        5 => null,
    ),
    6 => null,
);
function writeList($items){
    if($items === null)
        return;
    echo '<ul>';
    foreach($items as $item => $children){
        echo '<li>'.$item;
        writeList($children);
        echo '</li>';
    }
    echo '</ul>';
}
writeList($arr);