yii2验证码图像不显示

我在yii2中有以下代码,但验证码图像没有显示!
控制器:

 public function actions() {
    return [
        'captcha' => [
            'class' => 'yii\captcha\CaptchaAction',
            'fixedVerifyCode' => YII_ENV_TEST ? 'testme' : null,
            'foreColor' => 0xF9AF21,
            'maxLength' => 5,
            'minLength' => 3,
            'padding' => 5,
            'offset' => 1,
            'transparent' => true,
            'height' => 40
        ],
        'error' => [
            'class' => 'yii\web\ErrorAction',
        ],
    ];
}

型号:(规则)

['verifyCode', 'captcha',],

视图:

$form->field($model, 'verifyCode')->widget(Captcha::className()]) 

在SiteController中查找behavior()函数,它可能在下面的示例中看起来像.

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

你不会看到验证码图像,如果在你的behavior()函数中没有指定'only'动作,就像这样:'only' => ['logout', 'signup'],.该行表示仅对此操作应用访问规则.如果您不想为特定操作添加规则,则可以'captcha'向规则添加操作,如下例所示.

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