Description:
------------
The documentation for the return value of Iterator::key() states that it will be integer 0 on failure.

1. The docs also state that an E_WARNING will be issued. This is probably the only correct response.
2. Integer 0 is a valid key for an iterator so it should not be used as an error signal value.

Test script:
---------------
<?php

$x = array();
$x[] = 'zero';
$x[] = 'one';
$obj = new ArrayObject($x);
$iter = $obj->getIterator();

echo $iter->key() . PHP_EOL;
echo $iter->current() . PHP_EOL;

?>


Expected result:
----------------
0
zero


Actual result:
--------------
0
zero

[Correct output, but it doesn't match the docs since key() is not returning an error but it is returning 0.]