// scenario is set as a property

$model = new User;

$model->scenario = User::SCENARIO_SHOW;

// scenario is set through configuration

$model = new User(['scenario' => User::SCENARIO_SUBMIT]);

<?php

namespace app\models;

use yii\base\Model;

class ContactForm extends Model

{

    public $username;

    public $password;

    public $email;

    const SCENARIO_SHOW = 'show';

    const SCENARIO_SUBMIT = 'submit';

    public function scenarios()

    {

        return [

            self::SCENARIO_SHOW => ['username', 'password'],

            self::SCENARIO_SUBMIT => ['username', 'email', 'password'],

        ];

    }

    public function rules()

    {

        return [

            // username, email and password are all required in "submit" scenario

            [['username', 'email', 'password'], 'required', 'on' => self::SCENARIO_SUBMIT],

            // username and password are required in "show" scenario

            [['username', 'password'], 'required', 'on' => self::SCENARIO_SHOW],

        ];

    }

    public function attributeLabels()

    {

        switch($this->scenario)

        {

            case self::SCENARIO_SHOW:

                $labels = [

                    'username' => 'User Name',

                    'password' => 'Password',

                ];

                break;

            case self::SCENARIO_SUBMIT:

                $labels = [

                    'username' => 'User Name',

                    'password' => 'Password',

                    'email' => 'Your email address',

                ];

                break;

        }

        return $labels;

    }

}