I have an array that is structured like this:
[33] => Array
(
[time] => 1285571561
[user] => test0
)
[34] => Array
(
"SPL-way":
$splArray = SplFixedArray::fromArray($array);
$last_item_with_preserved_index[$splArray->getSize()-1] = $splArray->offsetGet($splArray->getSize()-1);
Read more about SplFixedArray and why it's in some cases ( especially with big-index sizes array-data) more preferable than basic array here => The SplFixedArray class.