Since PHP 5.5.X foreach can accept non scalar items. So the return can be anything ;)
(PHP 5, PHP 7)
Iterator::key — 返回当前元素的键
返回当前元素的键。
此函数没有参数。
成功返回标量，失败则返回 NULL。
失败时分发 E_NOTICE 级错误。
Since PHP 5.5.X foreach can accept non scalar items. So the return can be anything ;)
And converts everything to integer except string, so in php the post process could be:
public function key() {
$yourKey = $this->createYourKey();
if (is_object($yourKey) || is_array($yourKey))
throw new Exception('Array and Object not allowed.');
elseif (is_string($yourKey))
return $yourKey;
else
return (int) $yourKey;
}