public function validatePresenceOr($key1, $key2)
    {
        if ($this->exists($key1))
        {
            // Throw an exception if both parameters exist
            if ($this->exists($key2))
            {
                throw new \InvalidArgumentException("Either parameter '$key1' or '$key2' must be supplied, but not both.");
            }

            return true;
        }
        else if ($this->exists($key2))
        {
            return true;
        }

        throw new \InvalidArgumentException("Either the parameter '$key1' or '$key2' must be supplied.");
    }