respect-validation/library/Rules/Instance.php

<?php
namespace Respect\Validation\Rules;

class Instance extends AbstractRule
{
    public $instanceName;

    public function __construct($instanceName)
    {
        $this->instanceName = $instanceName;
    }

    public function reportError($input, array $extraParams = array())
    {
        return parent::reportError($input, $extraParams);
    }

    public function validate($input)
    {
        return $input instanceof $this->instanceName;
    }
}