GridView show sum of columns in footer
`PHP
use yii\grid\DataColumn;
/**

• Sum of all the values in the column
• @author shiv
  
  /
  class TSumColumn extends DataColumn {
  public function getDataCellValue($model, $key, $index)
  {
  $value = parent::getDataCellValue($model, $key, $index);
  if ( is_numeric($value))
  {
  $this->footer += $value;
  }

  return $value;
  

  
  }
  }
  `

Now you have to enable footer in GridView
echo GridView::widget([
'dataProvider' => $dataProvider,
'filterModel' => $searchModel,
'showFooter' => true,

Also change the coulmn class
[
'class' => TSumColumn::class,
'attribute' => 'amount'
],

You would see the total in footer of the grid. you can apply this to multiple columns if need