Codeigniter用户数据不可用

我正在为我的项目使用CodeIgniter.我快结束了，现在无法更改会话引擎".我有验证码图片，用于在网址上发布评论:

I'm using CodeIgniter for my project. I'm near end and I cannot change session "engine" now. I have captcha image for comment posting on url:

example.com/captcha

和代码:

this->load->library('session');
header("Content-type: image/jpeg");
/*
* Image generation code...
*/
$this->session->set_userdata('captcha', strtolower($word));
imagejpeg($image);
imagedestroy($image);

通常在我提交表单时使用

Usually when I'm submitting a form I use

<img src="example.com/captcha" />

生成图像并设置用户数据.但是这一次不起作用.$ this-> session-> userdata('captcha')为空.

to generate image and set userdata. But this time it doesn't work. $this->session->userdata('captcha') is empty.

但是，如果我直接访问图像，则会设置sessiondata，并在下次提交时直接在图像/验证码上生成一个$ word.是什么原因造成的?

But if I visit image directly sessiondata is set and on next submit there is a $word which was generated on direct access of image/captcha. What could cause this?