title: Yii2: Kartik Gridview sum of a column in footer
body: <p>I have installed the Kartik gridview extension, which is working fine.</p>
  
  <p>But I couldn't find or missed it in the docs, how I can show the sum of a column in the footer.</p>
  
  <p>This is my complete code in <code>index.php</code></p>
  
  <pre><code>&lt;?php 
          $gridColumns = [
      ['class' =&gt; 'yii\grid\SerialColumn'],
      'id',
      [
          //'attribute'=&gt;'service_name',
          'attribute'=&gt;'service_name',
          'value'=&gt;'serviceName.services',                
      ],
      [
          'attribute'=&gt;'room_category',
          'value'=&gt;'roomCategory.room_category'
       ],
          'charges_cash',
          'charges_cashless',
      ['class' =&gt; 'yii\grid\ActionColumn']
  ];
  
      echo ExportMenu::widget([
      'dataProvider' =&gt; $dataProvider,
      'columns' =&gt; $gridColumns,
      'fontAwesome' =&gt; true,
      'showPageSummary' =&gt; true,
      'dropdownOptions' =&gt; [
          'label' =&gt; 'Export All',
          'class' =&gt; 'btn btn-default'
      ]
  ]) 
          ?&gt;
      &lt;/div&gt;&lt;/div&gt;
  
  
      &lt;?= GridView::widget([
          'dataProvider' =&gt; $dataProvider,
          'filterModel' =&gt; $searchModel,
          'columns' =&gt; [
              ['class' =&gt; 'yii\grid\SerialColumn'],
  
              'id',
              //'service_name',
              [
                  //'attribute'=&gt;'service_name',
                  'attribute'=&gt;'service_name',
                  'value'=&gt;'serviceName.services',
  
              ],
             // 'room_category',
              [
                  'attribute'=&gt;'room_category',
                  'value'=&gt;'roomCategory.room_category'
              ],
              'charges_cash',
              'charges_cashless',
  
              ['class' =&gt; 'yii\grid\ActionColumn'],
          ],
           'showFooter' =&gt; true
      ]); ?&gt;
  
  &lt;/div&gt;
  </code></pre>
  
  <p>Looking for some help on this one.
  Thanks.</p>