use the 'CachingIterator::TOSTRING_USE_KEY', when echo this Object,
output current 'key';
<?php
$iterator = new ArrayIterator(array('apple'=>10, 'banana'=>20, 'cat'=>30));
$cache = new CachingIterator($iterator, CachingIterator::TOSTRING_USE_KEY);
$cache->next();
$cache->next();
echo $cache;
?>
output:
banana