ThinkPHP5如何实现验证码的效果

//显示验证码
    public function show_captcha(){
        $captcha = new \think\captcha\Captcha();
        $captcha->imageW=121;
        $captcha->imageH = 32;  //图片高
        $captcha->fontSize =14;  //字体大小
        $captcha->length   = 4;  //字符数
        $captcha->fontttf = '5.ttf';  //字体
        $captcha->expire = 30;  //有效期
        $captcha->useNoise = false;  //不添加杂点
        return $captcha->entry();
    }

<form action="/login/login_post" method="post">
<input type="text" class="input" name="captcha" placeholder="填写右侧的验证码" data-validate="required:请填写右侧的验证码"  style="width: 200px;"/>
                                <img src="/login/show_captcha" alt="" width="121" height="32" class="passcode" onclick="this.src=this.src+&#39;?&#39;"/>

//提交
    public function login_post(){
        $code=input('post.captcha');
        $captcha = new \think\captcha\Captcha();
        $result=$captcha->check($code);
        if($result===false){
            echo '验证码错误';exit;
        }
        echo '验证码正确，继续';exit;
    }