<?php

$array = array (
 '1' => 'Item',
 '2' => 'Item 2',
 '2.1' => 'Item 3',
 '2.2' => 'Item 4',
 '2.2.1' => 'Item 5',
 '2.2.1.1' => 'Item 6',
 '2.2.1.2' => 'Item 7',
);

$array_reversed = array_reverse($array, true);
$result = array();
$new_key = '';
foreach ($array_reversed as $key => $value) {
  if (!empty($new_key)) {
    $result[$new_key] = $value;
  }
  $new_key = $key;
}
$result = array_reverse($result, true);
print_r($result);

Array
(
    [2] => Item
    [2.1] => Item 2
    [2.2] => Item 3
    [2.2.1] => Item 4
    [2.2.1.1] => Item 5
    [2.2.1.2] => Item 6
)