public function authenticationUrl($redirect, $approvalPrompt = 'auto', $scope = null, $state = null)
        {
            $parameters = array(
                'client_id'       => $this->clientId,
                'redirect_uri'    => $redirect,
                'response_type'   => 'code',
                'approval_prompt' => $approvalPrompt,
                'state'           => $state,
            );

            if (! is_null($scope)) {
                $parameters['scope'] = $scope;
            }

            return $this->parseGet(
                $this->authUrl . 'authorize',
                $parameters
            );
        }