CachingIterator::offsetGet

offsetGet($index) returns the value stored at $index in the cache. Until you've 
iterated over the items, the cache is empty and the index will not exist.
 <?php
 $cache = new \CachingIterator(
 new \ArrayIterator(['a', 'b', 'c', 'd']),
 \CachingIterator::FULL_CACHE);
 $shortRange = range(0, 1);
 foreach ($shortRange as $index) {
 $cache->next();
 }
 echo PHP_EOL . 'The cache' . PHP_EOL;
 var_export($cache->getCache());
 echo PHP_EOL;
 echo $cache->offsetGet('1') . PHP_EOL;
 echo $cache->offsetGet('2') . PHP_EOL;
?>

The cache
array (
 0 => 'a',
 1 => 'b',
)
b
Undefined index: 2