AndrewCarterUK - PSR-6 getExpiration() solution

<?php

class Item
{
    private $key;
    private $expirations;

    public function __construct($key, ArrayObject $expirations)
    {
        $this->key = $key;
        $this->expirations = $expirations;
    }

    public function __destruct()
    {
        if (isset($this->expirations[$this->key])) {
            unset($this->expirations[$this->key]);
        }
    }

    public function getKey()
    {
        return $this->key;
    }

    public function setExpiration($expiration)
    {
        $this->expirations[$this->key] = $expiration;
    }
}

class Pool
{
    private $expirations;

    public function __construct()
    {
        $this->expirations = new ArrayObject();
    }

    public function getItem($key)
    {
        return new Item($key, $this->expirations);
    }

    public function save(Item $item)
    {
        $key = $item->getKey();

        if (isset($this->expirations[$key])) {
            echo 'Item ' . $key . ' expires at: ' . $this->expirations[$key] . PHP_EOL;
        } else {
            echo 'Item ' . $key . ' expiry not set' . PHP_EOL;
        }
    }
}

// Test it works

$pool = new Pool();

$foo = $pool->getItem('foo');
$bar = $pool->getItem('bar');

$foo->setExpiration(100);

$pool->save($foo); // Item foo expires at 100
$pool->save($bar); // Item bar expiry not set

// Check for protection against memory leak
$foo2 = clone $foo;
unset($foo);
$pool->save($foo2); // Item foo expiry not set