<?php

$i = new ArrayIterator(array(1,7,9));

$ci = new CachingIterator($i, CachingIterator::FULL_CACHE);

foreach($ci as $value) {
    echo $value . PHP_EOL;
    var_dump($ci->getCache());
}

1
array(1) {
[0]=>
int(1)
}
7
array(2) {
[0]=>
int(1)
[1]=>
int(7)
}
9
array(3) {
[0]=>
int(1)
[1]=>
int(7)
[2]=>
int(9)
}

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PHP 7.3, Zend 3.0, Symfony 4.3, Laravel 5.8, PHPUnit 8.0, Doctrine 2.10