<a href="javascript:void(0);" onclick="load_captcha(‘captcha‘,‘<?php echo site_url(‘index/show_captcha‘)?>‘);" title="看不清，换一张" id="captcha" ><?php echo $image;?></a>

function load_captcha(id,url)
{
 $("#"+id).html(‘‘);
 $("#"+id).load(url); 
}

Controller中方法

public function captcha()
	{
		$this->load->helper(‘captcha‘);
		$vals = array(
			‘word‘ => rand(1000,9999),
			‘img_path‘ => ‘./captcha/‘,
			‘img_url‘ => base_url().‘captcha/‘,
			‘font_path‘ => ‘./system/fonts/texb.ttf‘,
			‘img_width‘ => ‘90‘,
			‘img_height‘ => 35,
			‘expiration‘ => 7200
			);
		$this->session->set_userdata(array(‘word‘=>$vals[‘word‘]));
		$cap = create_captcha($vals);
		return $cap[‘image‘];	
	}
	
	public function show_captcha()
	{
		echo $this->captcha();
	}

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