Controller class for Yii 2 to support a wizard using one form model and several actions with branching logic.

WizardController.php

<?php

namespace spinitron\controllers;

use Yii;
use yii\web\Controller;
use yii\web\Session;

class WizardController extends Controller
{
    /**
     * @var \yii\base\Model Class name of the Wizard's form model
     */
    static $formClass;
    /**
     * @var string Route of the first form of the wizard.
     */
    static $startRoute = 'index';

    /**
     * @var Session
     */
    protected $session;
    /**
     * @var string
     */
    protected $step;
    /**
     * @var array
     */
    protected $attributes;

    public function beforeAction($action)
    {
        if (!parent::beforeAction($action)) {
            return false;
        }

        $this->session = Yii::$app->session;
        if (!$this->session->isActive) {
            $this->session->open();
        }

        if ($this->session->has('step')) {
            $this->step = $this->session->get('step');
            $this->attributes = $this->session->get('attributes');
        } else {
            // Just being extra careful...
            $this->session->remove('attributes');
        }

        return true;
    }

    /**
     * Clear session state and redirect to start of wizzard.
     */
    public function startOver()
    {
        $this->session->remove('step');
        $this->session->remove('attributes');
        $this->redirect([static::$startRoute]);
    }

    /**
     * Create a form model and process it for the $current step of the wizzard.
     *
     * Redirect to $next on success, return form model with errors on validation error.
     *
     * @param string $current Scenario of current action
     * @param string|callable|bool $next Route to redirect to on success. If callable, it
     * takes the form model as argument and returns a (string) route, or it can redirect.
     * Set false to return the $form without redirecting.
     *
     * @return \yii\base\Model Form model with validation errors
     */
    public function doForm($current, $next)
    {
        /** @var \yii\base\Model $form */
        $form = new static::$formClass(['scenario' => $current]);

        if ($this->attributes !== null) {
            foreach ($this->attributes as $key => $value) {
                $form->$key = $value;
            }
        }

        $loaded = $form->load(Yii::$app->request->post());
        if ($loaded && $form->validate()) {
            $this->session->set('attributes', $form->toArray());
            $this->session->set('step', $current);
            if ($next !== false) {
                $this->redirect([is_callable($next) ? call_user_func($next, $form) : $next]);
            }
        }

        return $form;
    }
}