thinkphp5 verification code use

simple controller

/**
     * Generate verification code
     * @param vid
     */
    public function verify()
    {
        $captcha = new \think\captcha\Captcha();
        $captcha->fontSize = 20;
        $captcha->length   = 4;
        $captcha->codeSet = '0123456789';
        $captcha->useNoise = false;
        return $captcha->entry();
    }


<div class="layui-form-mid">
    <span><img onclick= " this.src='{:url( " simple/verify " )}' " alt= " Click to replace " title= " Click to replace " src= " {:url('simple/verify' )} " >    
        </span>
</div>