Description:
------------
First,I want to know how to use __tostring.But there is not doc. So I tried with -3: 
and what I get:
Fatal error: Uncaught exception 'InvalidArgumentException' with message 'Flags must contain only one of CIT_CALL_TOSTRING, CIT_TOSTRING_USE_KEY, CIT_TOSTRING_USE_CURRENT' in CachingIterator::__tostring.php:12
Stack trace:
#0 CachingIterator::__tostring.php(12): CachingIterator->__construct(Object(ArrayIterator), -3)
#1 {main}
  thrown in CachingIterator::__tostring.php on line 12

So I know I should use CIT_CALL_TOSTRING, CIT_TOSTRING_USE_KEY or CIT_TOSTRING_USE_CURRENT.
then I tried:
$ak=(new CachingIterator($asx,CIT_TOSTRING_USE_KEY ));

But I again got an exception which prompted that the argument was still invalid.


Reproduce code:
---------------
<?php
error_reporting(E_ALL);
$asp=array("one"=>1,"two"=>2,"three"=>3,"four"=>4,"five"=>5,"six"=>6);
$kfc=array("trap");

$jsp=(new ArrayObject($asp));
$mcd=(new ArrayObject($kfc));

$asx=$jsp->getIterator();
$coca=$mcd->getIterator();

$ak=(new CachingIterator($asx,CIT_TOSTRING_USE_KEY ));
?>




Expected result:
----------------
no exception throwed out

Actual result:
--------------
Fatal error: Uncaught exception 'InvalidArgumentException' with message 'CachingIterator::__construct() expects parameter 2 to be long, string given' in CachingIterator::__tostring.php:12
Stack trace:
#0 CachingIterator::__tostring.php(12): CachingIterator->__construct(Object(ArrayIterator), 'CIT_TOSTRING_US...')
#1 {main}
  thrown in CachingIterator::__tostring.php on line 12