How to post actual primary keys of records from table and not indexes of an array in php Yii2 if I use select2 widget?

My function inside controller is as follows:

public function actionProjects($id)
    {
        $model = $this->findModel($id);
        $projects = Project::find()->all();
        $projectIds = Yii::$app->request->post('projects');
        if (Yii::$app->request->isPost) {
            Yii::$app->session->setFlash('success');
            $model->unlinkAll('projects');
            if ($projectIds) {
                foreach ($projectIds as $projectId) {
                    $project = Project::findOne(abs((int)$projectId));
                    if ($project) {
                        $model->link('projects', $project);
                    }
                }
            }
        }
        print_r($projectIds);
        return $this->render('projects', ['projects' => $projects, 'model' => $model]);
    }

My view is as follows:

<?php
...
function sortArray($a, $b)
{
    return strcmp($a, $b);
}

$projects = ArrayHelper::map($projects, 'id', 'title');
usort($projects, 'sortArray');
?>
<div class="application-create x_panel">

    <div class="x_title"><h3><?= Html::encode($this->title) ?></h3></div>

    <div class="x_content">
        <?php $form = ActiveForm::begin(); ?>
        <div class="form-group">
            <?= Select2::widget([
                'name' => 'projects[]',
                'options' => ['placeholder' => 'Выберите проект...'],
                'language' => 'ru',
                'value' => array_keys(ArrayHelper::map($model->projects, 'id', 'title')),
                'data' => $projects,
                'pluginOptions' => [
                    'allowClear' => true,
                    'multiple' => true
                ],
            ]) ?>
        </div>
        <div class="form-group">
            <?= Html::submitButton('Обновить', ['class' => 'btn btn-primary']) ?>
        </div>
        <?php ActiveForm::end() ?>
    </div>

</div>

When I select single or multiple items from select2, and click update button - this code:

$projectIds = Yii::$app->request->post('projects');

posts array like this: Array ( [0] => 0 [1] => 1 [2] => 2 )

the data sent to the server is the array of indexes of items and not actual indexes (primary keys) from the table. (I have 3 items in db table - their indexes are 5,6 and 7)

How to sent primary keys of selected items? I cannot find hidden rat that causes my problem. It seems that everything is correct in code. But actually not.

The problem is in the usort() call. That function removes the indexes in your $projects array. You have to use uasort() instead which is designed to maintain indexes in associative arrays.