protected function url($path, array $parameters = null)
    {
    	$query = [
            'client_id' => $this->client_key,
    	    'redirect_uri' => $this->redirect_uri,
    	    'response_type' => 'code'
    	];

        if ($parameters)
            $query = array_merge($query, $parameters);

        $query = http_build_query($query);

        return sprintf('%s%s?%s', self::API_HOST, $path, $query);
    }