$items = [
    ['label' => 'Home', 'url' => ['/site/index']],
    ['label' => 'About', 'url' => ['/site/about']],
    ['label' => 'Contact', 'url' => ['/site/contact']],
];

if (Yii::$app->user->isGuest) {
    $items[] = ['label' => 'Login', 'url' => ['/site/login']];
    $items[] = ['label' => 'Registration', 'url' => ['/site/registration']];
} else {
    $items[] =  '<li>'
                . Html::beginForm(['/site/logout'], 'post', ['class' => 'navbar-form'])
                . Html::submitButton(
                    'Logout (' . Yii::$app->user->identity->username . ')',
                    ['class' => 'btn btn-link']
                )
                . Html::endForm()
                . '</li>';
}

echo Nav::widget([
    'options' => ['class' => 'navbar-nav navbar-right'],
    'items' => $items,
]);