后台隶属于系统管理员登录范畴，此处使用admin表作为基础，其基础字段与user表保持一致。
后台登录模块采用独立模型和表单模型来完成。

1. 创建后台管理员表

详情参见：“账户管理”

2.修改后台主配置文件

/backend/config/main.php

'components' => [
    'request' => [
        'csrfParam' => '_csrf-backend',
    ],
    'user' => [
        'identityClass' => 'backend\models\Login',
        'enableAutoLogin' => true,
        'identityCookie' => ['name' => '_identity-backend', 'httpOnly' => true],
    ],
],

注意此处有一个user的键名，若修改为admin则会出现错误。

3 修改登录方法

/backend/controllers/SiteController.php

use backend\models\LoginForm;
public function actionLogin()
{
    if (!Yii::$app->user->isGuest) {
        return $this->goHome();
    }
    $model = new LoginForm();
    if ($model->load(Yii::$app->request->post()) && $model->login()) {
        return $this->goBack();
    } else {
        return $this->render('login', [
            'model' => $model,
        ]);
    }
}

4 创建模型

/backend/models/Login.php

<?php
namespace backend\models;
use Yii;
use yii\base\NotSupportedException;
use yii\behaviors\TimestampBehavior;
use yii\db\ActiveRecord;
use yii\web\IdentityInterface;

class Login extends ActiveRecord implements IdentityInterface
{
    const STATUS_DELETED = 0;
    const STATUS_ACTIVE = 10;
    public static function tableName()
    {
        return '{{%admin}}';
    }
    public function behaviors()
    {
        return [
            TimestampBehavior::className(),
        ];
    }
    public function rules()
    {
        return [
            ['status', 'default', 'value' => self::STATUS_ACTIVE],
            ['status', 'in', 'range' => [self::STATUS_ACTIVE, self::STATUS_DELETED]],
        ];
    }
    public static function findIdentity($id)
    {
        return static::findOne(['id' => $id, 'status' => self::STATUS_ACTIVE]);
    }
    public static function findIdentityByAccessToken($token, $type = null)
    {
        throw new NotSupportedException('"findIdentityByAccessToken" is not implemented.');
    }
    public static function findByUsername($username)
    {
        return static::findOne(['username' => $username, 'status' => self::STATUS_ACTIVE]);
    }
    public static function findByPasswordResetToken($token)
    {
        if (!static::isPasswordResetTokenValid($token)) {
            return null;
        }
        return static::findOne([
            'password_reset_token' => $token,
            'status' => self::STATUS_ACTIVE,
        ]);
    }
    public static function isPasswordResetTokenValid($token)
    {
        if (empty($token)) {
            return false;
        }
        $timestamp = (int) substr($token, strrpos($token, '_') + 1);
        $expire = Yii::$app->params['user.passwordResetTokenExpire'];
        return $timestamp + $expire >= time();
    }
    public function getId()
    {
        return $this->getPrimaryKey();
    }
    public function getAuthKey()
    {
        return $this->auth_key;
    }
    public function validateAuthKey($authKey)
    {
        return $this->getAuthKey() === $authKey;
    }
    public function validatePassword($password)
    {
        return Yii::$app->security->validatePassword($password, $this->password_hash);
    }
    public function setPassword($password)
    {
        $this->password_hash = Yii::$app->security->generatePasswordHash($password);
    }
    public function generateAuthKey()
    {
        $this->auth_key = Yii::$app->security->generateRandomString();
    }
    public function generatePasswordResetToken()
    {
        $this->password_reset_token = Yii::$app->security->generateRandomString() . '_' . time();
    }
    public function removePasswordResetToken()
    {
        $this->password_reset_token = null;
    }
}

/backend/models/LoginForm.php

<?php
namespace backend\models;
use Yii;
use yii\base\Model;

class LoginForm extends Model
{
    public $username;
    public $password;
    public $rememberMe = true;
    private $_admin;

    public function rules()
    {
        return [
            [['username', 'password'], 'required'],
            ['rememberMe', 'boolean'],
            ['password', 'validatePassword'],
        ];
    }

    public function validatePassword($attribute, $params)
    {
        if (!$this->hasErrors()) {
            $model = $this->getAdmin();
            if (!$model || !$model->validatePassword($this->password)) {
                $this->addError($attribute, 'Incorrect username or password.');
            }
        }
    }
    public function login()
    {
        if ($this->validate()) {
            return Yii::$app->user->login($this->getAdmin(), $this->rememberMe ? 3600 * 24 * 30 : 0);
        } else {
            return false;
        }
    }
    protected function getAdmin()
    {
        if ($this->_admin === null) {
            $this->_admin = Login::findByUsername($this->username);
        }
        return $this->_admin;
    }
}