how to display default value in dropDownlist - yii2

Question

I have two dropDownlist in index, when I select the submit button it will get action in controller, and it performs some operations and again render the index with gridview, but the Selected dropDownList become blank, how can I set the default in dropDownllist??
Any suggestion should be appreciatable...
this is my view
<div class="col-sm-12" align="center">
<?= $form->field($model, 'fk_int_payroll_month')->dropDownList(
ArrayHelper::map(TblPayrollMonth::find()->all(), 'pk_int_payroll_month_id','vchr_month'),
['prompt'=> 'Select...'])
?>
</div>
<div class="col-sm-12" align="center">
<?= $form->field($model, 'fk_int_payroll_year')->dropDownList(
ArrayHelper::map(TblPayrollYear::find()->all(), 'pk_int_payroll_year_id','year'),
['options' => [isset($_POST['fk_int_payroll_year'])?'fk_int_payroll_year':'' => ['Selected'=>true]]],
['prompt'=> 'Seect...'])
?>
</div>
this is my controller
public function actionDisplay()
{
$model = new TblPayroll();
if(Yii::$app->request->post()!=null)
{
$data = Yii::$app->request->post();
// var_dump($data);
// die;
$month = $data['TblPayroll']['fk_int_payroll_month'];
$year = $data['TblPayroll']['fk_int_payroll_year'];
/* Be careful with this! */
$dataProviderSearch = new ActiveDataProvider
([
'query' => TblPayroll::find()->where(['fk_int_payroll_month'=>$month, 'fk_int_payroll_year'=> $year]),
'pagination' => ['pageSize' => 5],
]);
if($dataProviderSearch)
{
return $this->render('index', [
'dataProviderSearch' => $dataProviderSearch,
'model' => $model,
]);
}
}
else{
return $this->render('index', ['model' => $model]);
}
when i select dropdown it will gott actionDisplay, and find the dataprovider, and then it again render to index, so that time how can i show default selected value in dropDownlist?

Answer 1

If you are using an activeRecord then in controllerAction you should assign to the $model->your_fiedl the value you need for default
else{
$model->fk_int_payroll_month = 3
return $this->render('index', ['model' => $model]);
}
if you don't are using an active record you could use
->dropDownList($yourlist ,$selection)
eg
->dropDownList(ArrayHelper::map(TblPayrollMonth::find()->all() ,3)