yii2自带验证码功能，配置一下就好了，这篇文章以登录页面为例，登录界面加入验证码可以防止暴力破解登录密码。
主要修改三个文件：LoginForm.php,siteController.php,login.php,
三个文件里面都引入下面类
use yii\captcha\CaptchaAction;
use yii\captcha\Captcha;

1.修改模型LoginForm类：

public $verifyCode;//给model增加一个属性 
public function rules()
    {
        return [
            // username and password are both required
            [['username', 'password','verifyCode'], 'required'],
            // rememberMe must be a boolean value
            ['rememberMe', 'boolean'],
            // password is validated by validatePassword()
            ['password', 'validatePassword'],
            ['verifyCode', 'captcha'],
        ];
    }
public function attributeLabels() 
    { 
         return [ 
                   // 'verifyCode' => 'Verification Code', 
                   'username' => '用户名',
                   'password'=>'密码',
                   'verifyCode' => '验证码',
            ]; 
    } 

2.修改SiteController.php

public function behaviors()
    {
        return [
            'access' => [
                'class' => AccessControl::className(),
                'only' => ['logout', 'signup','login'],
                'rules' => [
                    [
                        'actions' => ['signup','login','captcha'],
                        'allow' => true,
                        'roles' => ['?'],
                    ],
                    [
                        'actions' => ['logout'],
                        'allow' => true,
                        'roles' => ['@'],
                    ],
                ],
            ],
            'verbs' => [
                'class' => VerbFilter::className(),
                'actions' => [
                    'logout' => ['post'],
                ],
            ],
        ];
    }

    /**
     * {@inheritdoc}
     */
    public function actions()
    {
        return [
            'error' => [
                'class' => 'yii\web\ErrorAction',
            ],
            /*'captcha' => [
                'class' => 'yii\captcha\CaptchaAction',
                'fixedVerifyCode' => YII_ENV_TEST ? 'testme' : null,
            ],*/
            'captcha' => [
                                    'class' => 'yii\captcha\CaptchaAction',
                                    'fixedVerifyCode' => YII_ENV_TEST ? 'testme' : null,
                                    'backColor'=>0x000000,//背景颜色
                                    'maxLength' => 6, //最大显示个数
                                    'minLength' => 5,//最少显示个数
                                    'padding' => 5,//间距
                                    'height'=>40,//高度
                                    'width' => 130,  //宽度  
                                    'foreColor'=>0xffffff,     //字体颜色
                                    'offset'=>4,        //设置字符偏移量 有效果
                                    //'controller'=>'login',        //拥有这个动作的controller
                            ],
        ];
    }

3.修改view里的login.php

use yii\captcha\Captcha;
<?= $form->field($model, 'verifyCode')->widget(Captcha::className()) ?>

搞定！注册页面加入验证码可以防止恶意注册，原理一样，有兴趣的读者可以自行搞定！