//控制器SiteController
class SiteController extends Controller
{
    // ...
    public function actions()
    {
        return [
            // ...
            'captcha' => [
                'class' => 'yii\captcha\CaptchaAction',
                'fixedVerifyCode' => YII_ENV_TEST ? 'testme' : null,
            ],
        ];
    }

    // ...
}
?>
定义表单Model
class ContactForm extends Model
{
    // ...
    public $verifyCode;

    // ...
    public function rules()
    {
        return [
            // ...
            ['verifyCode', 'captcha'],
        ];
    }

    // ...
}
?>
在view中调用方法
$form = ActiveForm::begin(['id' => 'contact-form']); ?>
    // ...
$form->field($model, 'verifyCode')->widget(Captcha::className()) ?>
    // ...
ActiveForm::end(); ?>