offsetGet($index) 返回存储在缓存中 $index 处的 value。在您迭代完项目之前，缓存为空，索引将不存在。
<?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