application/extra/captcha.php

<?php
// +----------------------------------------------------------------------
// | ThinkPHP [ WE CAN DO IT JUST THINK IT ]
// +----------------------------------------------------------------------
// | Copyright (c) 2006-2016 http://thinkphp.cn All rights reserved.
// +----------------------------------------------------------------------
// | Licensed ( http://www.apache.org/licenses/LICENSE-2.0 )
// +----------------------------------------------------------------------
// | Author: yunwuxin <448901948@qq.com>
// +----------------------------------------------------------------------

return [
    'codeSet'  => '1234567890',
    'length'   => 4,
    'fontSize' => 16,
    'secure'=>false,
    'useNoise' => false,
    'useCurve' => false,
];