<?php
$ao = new ArrayObject();
$swapIn = array();
$ao->exchangeArray($swapIn);

$ao['a'] = 'This affects $swapIn';
$swapIn['b'] = 'This forces $swapIn to split from its copy-on-write references';
$ao['c'] = 'This no longer affects $swapIn';

var_dump($swapIn, $ao);

//Actual output on PHP 5.3.0-dev (cli) (built: Jul 28 2008 00:21:59) :
//
//array(2) {
//  ["a"]=>
//  string(20) "This affects $swapIn"
//  ["b"]=>
//  string(62) "This forces $swapIn to split from its copy-on-write references"
//}
//object(ArrayObject)#1 (1) {
//  ["storage":"ArrayObject":private]=>
//  array(2) {
//    ["a"]=>
//    string(20) "This affects $swapIn"
//    ["c"]=>
//    string(30) "This no longer affects $swapIn"
//  }
//}
?>