Iterator::key

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;
 }

This function may return any type, not just scalar, for some Iterator types. In particular, it is very trivial to write a generator function that yields arbitrary keys:
<?php
function foo() {
 yield null => 1;
 yield new stdclass => 2;
}
?>