_challengeClient example

 'authenticate()');
        }

        if ($this->_imaProxy) {
            $getHeader = 'Proxy-Authorization';
        } else {
            $getHeader = 'Authorization';
        }

        $authHeader = $this->_request->getHeader($getHeader);
        if (!$authHeader) {
            return $this->_challengeClient();
        }

        list($clientScheme) = explode(' ', $authHeader);
        $clientScheme = strtolower($clientScheme);

        // The server can issue multiple challenges, but the client should         // answer with only the selected auth scheme.         if (!in_array($clientScheme, $this->_supportedSchemes)) {
            $this->_response->setStatusCode(400);
            return new Zend_Auth_Result(
                Zend_Auth_Result::FAILURE_UNCATEGORIZED,