count.php

<?php

use yii\helpers\Html;
use yii\widgets\Pjax;
?>

<?php Pjax::begin(); ?>
<?= Html::a("Оновити", ['pjax/count', 'counter' => $counter], ['class' => 'btn btn-lg btn-primary']) ?>
    <h1>Лічильник: <?= $counter ?></h1>
<?php Pjax::end(); ?>

get-full-name.php

<?php

use yii\helpers\Html;
use yii\widgets\Pjax;
use yii\widgets\ActiveForm;
?>

<?php Pjax::begin(); ?>
<?php $form = ActiveForm::begin([
    'options' => ['data' => ['pjax' => true]],
]); ?>

<?= $form->field($model, 'first_name')->textInput() ?>
<?= $form->field($model, 'last_name')->textInput() ?>

    <div class="form-group">
        <div class="col-lg-offset-1 col-lg-11">
            <?= Html::submitButton('OK', ['class' => 'btn btn-primary']) ?>
        </div>
    </div>
<?php ActiveForm::end(); ?>
<?= $fullName; ?>
<?php Pjax::end(); ?>

get-server-time.php

<?php

use yii\helpers\Html;
use yii\widgets\Pjax;
?>

<?php Pjax::begin(); ?>
<?= Html::a("Оновити", ['pjax/get-server-time'], ['class' => 'btn btn-lg btn-primary']) ?>
  <h1>Зараз: <?= $time ?></h1>
<?php Pjax::end(); ?>

PjaxController.php

<?php


namespace app\controllers;


use app\models\Continent;
use app\models\StudentForm;
use app\models\WorldForm;
use Yii;
use yii\web\Controller;

class PjaxController extends Controller
{
    public function actionGetServerTime()
    {
        return $this->render('get-server-time', ['time' => date("h:i:s")]);
    }

    public function actionCount($counter = 0)
    {
        $counter++;
        return $this->render('count', compact('counter'));
    }

    public function actionGetFullName()
    {
        $model = new StudentForm();
        if (Yii::$app->request->isPjax) {
            $model->load(Yii::$app->request->post());
            $fullName = $model->first_name . " " . $model->last_name;
            return $this->render('get-full-name', compact('model', 'fullName'));
        }
        return $this->render('get-full-name', compact('model'));
    }

    public function actionWorldForm()
    {
        $model = new WorldForm();
        $continents = Continent::find()->asArray()->all();
        $continent = null;

        if (Yii::$app->request->isPjax) {
            $model->load(Yii::$app->request->post());
            $continent = Continent::find()->where(['continent_id' => $model->continent_id])->one();
        }
        return $this->render('world-form', [
            'model' => $model,
            'continents' => $continents,
            'continent' => $continent
        ]);
    }
}

StudentForm.php

<?php


namespace app\models;


use yii\base\Model;

class StudentForm extends Model
{
    public $first_name = "";
    public $last_name = "";

    public function rules()
    {
        return [
          [['first_name', 'last_name'], 'required']
        ];
    }

    public function attributeLabels()
    {
        return [
            'first_name' => "First Name",
            'last_name' => "Surname"
        ];
    }
}

world-form.php

<?php

use yii\helpers\ArrayHelper;
use yii\helpers\Html;
use yii\helpers\Url;
use yii\widgets\Pjax;
use yii\widgets\ActiveForm;
?>

<?php Pjax::begin([
    'id' => 'world-form-container'
]); ?>
<?php $form = ActiveForm::begin([
    'options' => ['data' => ['pjax' => true],],
    'id' => 'world-form'
]); ?>

<?= $form->field($model, 'continent_id')->dropDownList(ArrayHelper::map($continents, 'continent_id', 'name'), [
    'id' => 'field-continent-id',
    'onchange' => '$("#world-form").submit()'
]) ?>
<h2><?= $continent->name ?></h2>
<p><?= $continent->description ?></p>

<?php ActiveForm::end(); ?>
<?php Pjax::end(); ?>

WorldForm.php

<?php


namespace app\models;


use yii\base\Model;

class WorldForm extends Model
{
    public $continent_id = 0;
    public $country_id = 0;

    public function rules()
    {
        return [
            [['country_id', 'continent_id'], 'integer']
        ];
    }

    public function attributeLabels()
    {
        return [
            'continent_id' => "Continent",
            'country_id' => "Country",
        ]; // TODO: Change the autogenerated stub
    }
}