YII2 ขอความช่วยเหลือเรื่องการนำค่าจากฐานข้อมูลมาคำนวนเพื่อแสดงผล

<?php

use yii\helpers\Html;
use yii\grid\GridView;

$this->title = 'PLOY REQUEST MAK MAK';
$this->params['breadcrumbs'][] = $this->title;
?>
<div class="ldrequest-index">

    <h1><?= Html::encode($this->title) ?></h1>    
    <?= GridView::widget([
        'dataProvider' => $dataProvider,
        'filterModel' => $searchModel,
        'columns' => [
            ['class' => 'yii\grid\SerialColumn'],
            'DOC_NO',
            'PRIORITY',
            'REQUESTED_ON',
            'REQUESTED_NOTE',
            'PARTNUMBER',
            'DESCRIPTN',
            'QREQ',
            'QORDER',
            'QTY_UM',
            'REQUESTED_BY',

                        [
                          'class' => 'yii\grid\ActionColumn',
                         'buttonOptions'=>['class'=>'btn btn-default'],
                         'contentOptions'=>['noWrap' => true],
                         'template'=>'<div class="btn-group btn-group-lg text-center" role="group"> {view} </div>',
                         'options'=> [ 'style'=>'width:65px;']
                        ],

                        [
                          'header' => 'Days Work',
                          'value' => function () {
                              $cerrent= new DateTime('now');
                              $REQUESTED_NO= new DateTime('10-08-2018');                          
 
                              $interval = $cerrent->diff($REQUESTED_NO)->days;
                              return $interval;                
 
                          },
                      ],
                    ],
        ]); ?>
</div>