How to generate verification code using Zend_Captcha

class TestController extends Lyw0301_Controller_Action {
 public function init() {
  parent::init();
  $this->view->title = '测试';
  $this->view->baseUrl = $this->getFrontController()->getBaseUrl();
  // $this->_helper->viewRenderer->setNoRender();
  //Zend_Layout::getMvcInstance()->disableLayout();
 }
 function generateCaptcha() {
  $captcha = new Zend_Captcha_Image();
  $captcha->setTimeout('300')
  ->setWordLen('6')
  ->setHeight('80')
  ->setFont('./images/font/micross.ttf')
  ->setImgDir('./images/code');

  $captcha->generate();
  return $captcha->getId();
 }
 //validates captcha response
 function validateCaptcha($captcha) {
  $captchaId = $captcha['id'];
  $captchaInput = $captcha['input'];
  $captchaSession = new Zend_Session_Namespace('Zend_Form_Captcha_' . $captchaId);
  $captchaIterator = $captchaSession->getIterator();
  Zend_Debug::dump($captchaIterator);exit;
  $captchaWord = $captchaIterator['word'];
  if($captchaWord) {
   if( $captchaInput != $captchaWord ){
    return false;
   } else {
    return true;
   }
  } else {
   return false;
  }
 }
 public function indexAction() {
  $captchaId = $this->generateCaptcha();
  $this->view->captchaId = $captchaId;
  if(isset($_POST['captcha'])) {
   $captcha = $_POST['captcha'];
   if( $this->validateCaptcha($captcha) ) {
    $this->view->message = 'yes';
   } else {
    $this->view->message = 'no';
   }
  }
 }
}
?>