<form action="<{:url('index/index/login')}>" method="post" name="form1" >

　　<input type="text" name="username"><br>
　　<input type="text" name="password"><br>
　　<input type="text" name="captcha" placeholder="请输入验证码" required=""><br>
　　<img src="<{:captcha_src()}>" width="95" height="45" onclick="this.src='<{:captcha_src()}>'"><br><br>

　　<input type="submit"><br>

</form>

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$captcha = $_REQUEST['captcha'];

if(!captcha_check($captcha)){
　　//验证失败
　　$this->error('验证码输入错误');
}else{

　　$this->success('验证码输入正确');

}