how to keep selected value on yii2 html::dropddownlist? : i have the following code in my view:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-html -->

    <?=Html::dropdownList('region',null,
    ArrayHelper::map(Ethioregion::find()->all(),'region','region'),[
            'prompt' => 'Select Region..','style'=>'width:200px',]) ?>

<!-- end snippet -->
and submit button

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-html -->

     <?= Html::submitButton('Search', ['name' => 'dele','class' => 'btn btn-primary']) ?>

<!-- end snippet -->

my code working well but i have one big problem.
i select region and click search button, the result comes correctly, and 10 result is shown per page, when i click the next page, it gives me  false data, i notice this is due to the selected value on the dropdownlist is resetted. i tried both making " Selected'=>true  , and  'Selected'=>'selected', but it doestnot working. so how can i make the selected value on dropdownlist and checkboxlist will be selected after the page is loading?
thanks
