yii2 rules验证规则，ajax验证手机号码是否唯一

<?php

namespace frontend\models;

use Yii;
use yii\base\Model;

/**
 * Signup form
 */
class SignupForm extends Model
{
    public $mobile;
    public $captcha;
    public $password;

    /**
     * @inheritdoc
     */
    public function rules()
    {
        return [
            [['mobile', 'captcha', 'password'], 'trim'],
            [['mobile', 'captcha', 'password'], 'required'],

            [['mobile'], 'match', 'pattern' => '/^1[3|4|5|7|8][0-9]{9}$/'],
            [['mobile'], 'unique', 'targetClass' => '\common\models\User', 'message' => '该手机号已被注册！'],

            [['password'], 'match', 'pattern' => '/^\S+$/'],
            [['password'], 'string', 'length' => [6, 32]],

            [['captcha'], 'captcha'],
        ];
    }

    /**
     * @inheritdoc
     */
    public function attributeLabels()
    {
        return [
            'mobile' => '手机号',
            'captcha' => '验证码',
            'password' => '密码'
        ];
    }

    public function writeSession()
    {
        $session = Yii::$app->session;
        $session->open();
        $session['mobileSignupTimeout'] = time() + 600;
        $session['mobileSignup'] = $this->mobile;
        $session['mobileSignupPassword'] = $this->password;
    }
}

UserController.php

        if ($step !== '2') {
            $model = new SignupForm();
            $model->load(Yii::$app->request->post());

            if (Yii::$app->request->isAjax) {
                Yii::$app->response->format = Response::FORMAT_JSON;
                return ActiveForm::validate($model);
            }

            if (Yii::$app->request->isPost && $model->validate()) {
                $model->writeSession();
                if ($this->_sendMsg($model->mobile)) {
                    Yii::info("用户注册发送短信验证码成功！手机号：{$model->mobile}");
                    Yii::$app->session->setFlash('sentSuccess');
                } else {
                    Yii::warning("用户注册发送短信验证码失败！手机号：{$model->mobile}，说明：" . Yii::$app->smser->message);
                    Yii::$app->session->setFlash('failedToSend', '验证码发送失败，请您再试一次！');
                }
                return $this->redirect(['signup', 'step' => '2']);
            }

signup.php

        if ($step !== '2') {
            $model = new SignupForm();
            $model->load(Yii::$app->request->post());

            if (Yii::$app->request->isAjax) {
                Yii::$app->response->format = Response::FORMAT_JSON;
                return ActiveForm::validate($model);
            }

            if (Yii::$app->request->isPost && $model->validate()) {
                $model->writeSession();
                if ($this->_sendMsg($model->mobile)) {
                    Yii::info("用户注册发送短信验证码成功！手机号：{$model->mobile}");
                    Yii::$app->session->setFlash('sentSuccess');
                } else {
                    Yii::warning("用户注册发送短信验证码失败！手机号：{$model->mobile}，说明：" . Yii::$app->smser->message);
                    Yii::$app->session->setFlash('failedToSend', '验证码发送失败，请您再试一次！');
                }
                return $this->redirect(['signup', 'step' => '2']);
            }