Multi Dimensional Array as Nested List

<?php
$a=array (
    '0' => array (
        'id' => 1,
        'title' => "Title 1",
        'parent_id' => 'NULL',
        'depth' => 0
    ),
    '1' => array (
        'id' => 2,
        'title' => "Title 2",
        'parent_id' => 'NULL',
        'depth' => 0
    ),
    '2' => array (
        'id' => 3,
        'title' => "Title 3",
        'parent_id' => 2,
        'depth' => 1
    ),
    '3' => array (
        'id' => 4,
        'title' => "Title 4",
        'parent_id' => 2,
        'depth' => 1
    ),
    '4' => array (
        'id' => 5,
        'title' => "Title 5",
        'parent_id' => 'NULL',
        'depth' => 0
    ),
    '5' => array (
        'id' => 6,
        'title' => "Title 6",
        'parent_id' => 4,
        'depth' => 0
    ));
Here the code:

$level = 'NULL';

function r( $a, $level) {
  $r = "<ol>";
  foreach ( $a as $i ) {
      if ($i['parent_id'] == $level ) {
          $r = $r . "<li>" . $i['title'] . r( $a, $i['id'] ) . "</li>";
      }
  }
  $r = $r . "</ol>";
  return $r;}

print r( $a, $level );

?>

<ol><li>Title 1<ol></ol></li><li>Title 2<ol><li>Title 3<ol></ol></li><li>Title 4<ol><li>Title 6<ol></ol></li></ol></li></ol></li><li>Title 5
<ol></ol></li></ol>

function r( $a, $level) {
  $r = '' ;
  foreach ( $a as $i ) {
      if ($i['parent_id'] == $level ) {
          $r = $r . "<li>" . $i['title'] . r( $a, $i['id'] ) . "</li>";
      }
  }
  return ($r==''?'':"<ol>". $r . "</ol>");}