Retrieving data from Gridview as array in Yii2 - yii2

Question

i'd like to ask how to retrieving data as array from gridview
as mentioned in this thread
How can I get the selected data/item rows in CheckboxColumn Gridview - Yii2
what i have been doing is :
in the controller
public function actionCetakdispo{
$action=Yii::$app->request->post('action');
$selection=(array)Yii::$app->request->post('selection');
print_r($selection);
exit;
}
in view :
<?=Html::beginForm(['controller/rekapsm'],'post');?>
<?= Html::a('Cetak Dispo', ['cetakdispo'], ['class' => 'btn btn-
primary']) ?>
<?= Html::a('Cetak Register', ['cetakreg'], ['class' => 'btn btn-
primary']) ?>
<?= GridView::widget([
'dataProvider' => $dataProvider,
'filterModel' => $searchModel,
'columns' => [
['class' => 'yii\grid\SerialColumn'],
'id',
'kategori',
'nama_wp',
'nama',
'nomor_surat',
'tgl_surat',
'perihal',
'ket',
['class' => 'yii\grid\CheckboxColumn'],
],
]); ?>
but nothing data retrieve to array..please give me lessons or referrence to read..many thanks..
UPDATED :
i change view become likethis :
<div class="row" style="margin-left:870px;" >
<?=Html::beginForm(['cetakdispo'],'post');?>
<?=Html::submitButton('Cetak Dispo', ['class' => 'btn btn-primary']);?>
<?= Html::a('Cetak Register', ['cetakreg'], ['class' => 'btn btn-primary']) ?>
</div>
<?php // echo $this->render('_search', ['model' => $searchModel]); ?>
<?= GridView::widget([
'dataProvider' => $dataProvider,
'filterModel' => $searchModel,
'columns' => [
['class' => 'yii\grid\SerialColumn',],
'id',
'kategori',
'nama_wp',
'nama',
'nomor_surat',
'tgl_surat',
'perihal',
'ket',
['class' => 'yii\grid\CheckboxColumn', 'checkboxOptions' => function($model, $key, $index, $widget) {
return ['value' => $model['id'] ];
},],
],
]); ?>
<?= Html::endForm();?>
</div>
and its works...