请教一个Zend Framework中ajax验证码的问题

[zxywd]

各位下午好。我遇到一个很头疼的问题希望大家帮忙解决一下。是关于Zend Framework中ajax验证码的问题。代码如下：
首先，我修改了library/Zend/Captcha/Image.php文件中的render方法：

  public function render(Zend_View_Interface $view = null, $element = null)
    {
 		return '<img id="changeCaptcha" alt="'.$this->getImgAlt().'" src="' . $this->getImgUrl() . $this->getId() . $this->getSuffix() . '" style="cursor: pointer;" /><br/>';
    }

IndexController.php

<?php 
class IndexController extends Zend_Controller_Action   
{     
    public function indexAction()   
    {   
        $form = new Zend_Form();   
        $captcha = $this->createCaptcha();   
        $form->addElement($captcha);   
        $this->view->captchaImageUrl = $captcha->getCaptcha()->getImgUrl()   
                                     . $captcha->getCaptcha()->getId()   
                                     . $captcha->getCaptcha()->getSuffix();   
    
        $this->view->formLogin = $form->render();   
    }   
    
    public function ajaxAction()   
    {   
        $captcha = $this->createCaptcha()->getCaptcha();   
        $captcha->generate();
        $this->_helper->viewRenderer->setNoRender(true);   
        echo $captcha->getImgUrl() . $captcha->getId() . $captcha->getSuffix();   
        die; 
    }   
    
    public function createCaptcha()   
    {     
        $decorators = array(   
            array('HtmlTag', array('tag' => 'div',   
                                   'id' => 'captchaId',   
                                   'onclick' => 'changeImage()'))   
        );   
    
        $form = new Zend_Form();   
    
        $captcha = $form->createElement('captcha', 'captcha', array(   
            'captcha' => array(     
                'captcha' => 'Image',   
                'wordLen' => 6,   
                'fontsize' => 20,   
                'width' => 200,   
                'height' => 100,   
                'dotNoiseLevel' => 2,   
                'timeout' => 300,     
				'imgUrl'=>'/images',
				'imgDir'=>APPLICATION_PATH.'/../public/images',
				'font'=>APPLICATION_PATH.'/../public/images/LiberationSansRegular.ttf'  
            ),   
            'decorators' => $decorators   
        ));   
    
        return $captcha;   
    }   
}   

index.phtml

<script type="text/javascript">
function createXmlHttpObject()
{
	try 
	{ 
		xmlhttp = new XMLHttpRequest(); 
	} catch (trymicrosoft) { 
		try { 
			xmlhttp = new ActiveXObject("Msxml2.XMLHTTP"); 
		} catch (othermicrosoft) { 
			try { 
				xmlhttp = new ActiveXObject("Microsoft.XMLHTTP"); 
			} catch (failed) { 
				xmlhttp = false; 
			} 
		} 
	} 
	if (!xmlhttp){ 
		alert("无法创建 XMLHttpRequest 对象！"); 
	}
	return xmlhttp;
}

var xmlhttp = createXmlHttpObject();
//JavaScript Document
function $(id){
	return document.getElementById(id);
}

function changeImage()
{
	if(document.activeElement.id=='captcha-input')
	{
		return false;
	}
	url="/index/ajax";
	xmlhttp.open('GET',url,true);
	xmlhttp.onreadystatechange=function(){
		if(xmlhttp.readyState==4){
			if(xmlhttp.status==200)
			{
				response=xmlhttp.reponseText;
				$('changeCaptcha').src=response;
			}
		}
	};
}
</script>


<?php 
	echo $this->formLogin; 

当我用鼠标单击图片区域时，应该会显示新的验证码，不知道为什么，就是不能实现动态验证码的效果。希望各位能帮忙解决一下谢谢了。

[zxywd]

library/Zend/Captcha/Image.php文件中的render方法：刚才没显示清楚，现在重新写一下。

public function render(Zend_View_Interface $view = null, $element = null)
{
return '<img id="changeCaptcha" alt="'.
$this->getImgAlt().'" src="' . $this->getImgUrl() .
$this->getId() . $this->getSuffix() .
'" style="cursor: pointer;" /><br/>';
}