Typova kontrola v PHP kolekcii

namespace System\Collections;

class Collection extends \ArrayObject
{
    private $itemType = null;
    function setItemType($value) { $this->itemType = $value; }
    function getItemType() { return $this->itemType; }

    function offsetSet($index, $newval)
    {
        if (is_null($this->itemType)) parent::offsetSet($index, $newval);
        else
        {
            if ($newval instanceof $this->itemType) parent::offsetSet($index, $newval);
            else throw new InvalidTypeException;
        }
    }
}

class InvalidTypeException extends \Exception
{
}