gregwar/captcha 自定义验证码的位数和内容
// Will build phrases of 4 characters, only digits
$phraseBuilder = new PhraseBuilder(4, '0123456789');
$captcha_builder = new CaptchaBuilder(null,$phraseBuilder);
$captcha = $captcha_builder->build();
实例化一个 PhraseBuilder() 就行
gregwar/captcha 文档
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