array_wrapper.php

<?php

namespace acme;

function array_key_exists($key, $a) {
  if ($a instanceof \ArrayAccess) {
    return $a->offsetExists($key);
  } else {
    return \array_key_exists($key, $a);
  }
}

class ArrayWrapper extends \ArrayObject {
  public function offsetExists($key) {
    echo "this will be printed\n";
    return parent::offsetExists($key);
  }
}

$a = new ArrayWrapper(array('foo' => 1));

var_dump(array_key_exists('foo', $a)); // bool(true)
var_dump(array_key_exists('bar', $a)); // bool(false)