<?php

$results = array();

$obj = new ArrayObject();
$obj['hello'] = NULL;
$results['ArrayObject'] = array(
  'empty' => empty($obj['hello']),
  'isset' => isset($obj['hello']),
);

class ArrayObjectOverride extends ArrayObject {

  public function offsetExists($index) {
    return parent::offsetExists($index);
  }

}

$obj = new ArrayObjectOverride();
$obj['hello'] = NULL;
$results['ArrayObjectOverride'] = array(
  'empty' => empty($obj['hello']),
  'isset' => isset($obj['hello']),
);

header('Content-Type: text/plain');
var_dump($results);