EntityCollection::getIndexById

public function getIndexById($id)
{
	if (intval($id) <= 0)
	{
		throw new MainArgumentNullException('id');
	}

	/** @var CollectableEntity $item */
	foreach ($this->collection as $item)
	{
		if ($item->getId() > 0 && $id == $item->getId())
		{
			return $item->getInternalIndex();
		}
	}
	return null;
}