Reset the Internal Pointer Value of a PHP Array (ArrayObject)

Consider a simple PHP ArrayObject with two items.

$ao = new ArrayObject();
$ao[] = 'a1';  // [0] => a1
$ao[] = 'a2';  // [1] => a2

Then delete the last item and add a new item.

$ao->offsetUnset(1);
$ao[] = 'a3';  // [2] => a3

I'd very much like to be able to have 'a3' be [1].

How can I reset the internal pointer value before I add 'a3'?

I have a simple function that does this but I'd rather not copy the array if I don't have to.

function array_collapse($array) {
    $return = array();
    while ($a = current($array)) {
        $return[] = $a;
        next($array);
    }
    return $return;
}

With the expansion on the question in your comments: you'd have to extend the ArrayObject class to get this kind of behavior:

class ReindexingArray extends ArrayObject {
    function offsetUnset($offset){
        parent::offsetUnset($offset);
        $this->exchangeArray(array_values($this->getArrayCopy()));
    }
    //repeat for every other function altering the values.
}

Another option would be the SplDoublyLinkedList:

<?php
$u = new SplDoublyLinkedList();
$array = array('1' => 'one',
               '2' => 'two',
               '3' => 'three');
foreach($array as $value) $u[] = $value;
var_dump($u);
unset($u[1]);        
var_dump($u);
$u[] = 'another thing';
var_dump($u);

Why not use offsetSet:

$ao = new ArrayObject();
$ao[] = 'a1';  // [0] => a1
$ao[] = 'a2';  // [1] => a2
$ao->offsetUnset(1);
$ao->offsetSet(1, 'a3');

This is kind of lame, but you can cast it to standard array and use array_splice on it:

$ao = new ArrayObject();
$ao[] = 'element 1';
$ao[] = 'element 2';
$ao[] = 'element 3';
$ao[] = 'element 4';

var_dump($ao);

$items = (array) $ao;
array_splice($items, 1, 2);
$ao = new ArrayObject($items);
$ao[] = 'element 5';

var_dump($ao);

This results in:

object(ArrayObject)#1 (1) {
  ["storage":"ArrayObject":private]=>
  array(4) {
    [0]=>
    string(9) "element 1"
    [1]=>
    string(9) "element 2"
    [2]=>
    string(9) "element 3"
    [3]=>
    string(9) "element 4"
  }
}
object(ArrayObject)#2 (1) {
  ["storage":"ArrayObject":private]=>
  array(3) {
    [0]=>
    string(9) "element 1"
    [1]=>
    string(9) "element 4"
    [2]=>
    string(9) "element 5"
  }
}