<?php

use yiihelpersHtml;
use yiiwidgetsActiveForm;

$form = ActiveForm::begin([
    'id' => 'form1',
]) ?>
<?= $form->field($postForm, 'title') ?>
<?= $form->field($postForm, 'text') ?>
<?= $form->field($postForm, 'userId') ?>

<div class="form-group">
    <?= Html::submitButton('Добавить', ['class' => 'btn btn-primary']) ?>
</div>
<?php ActiveForm::end() ?>

public function actionAdd()
    {
        $postForm = new Post();

        if (Yii::$app->request->isAjax) {
            $post = new Post();
            $post->title = Yii::$app->request->post('title');
            $post->text = Yii::$app->request->post('text');
            $post->userId = Yii::$app->request->post('userId');
            $post->save();

            if (!$post->save()) {
                print_r($post->errors);
            }
        }

        return $this->render('addPost', compact('postForm'));
    }