Переделал виджет, более читаем

backend/modules/reports/views/ajax/_gridView.php

@@ -0,0 +1,34 @@
+<?php
+
+use yii\grid\GridView;
+
+/* @var $this yii\web\View */
+/* @var $dataProvider yii\data\ActiveDataProvider */
+
+echo GridView::widget([
+    'dataProvider' => $dataProvider,
+    'columns' => [
+        ['class' => 'yii\grid\SerialColumn'],
+        [
+            'attribute' => 'today',
+            'format' => 'raw',
+            'value' => function ($model) {
+
+                $text = '';
+                if ($model->task) {
+                    $i = 1;
+                    foreach ($model->task as $task) {
+                        $text .= "<p>$i. ($task->hours_spent ч.) $task->task</p>";
+                        $i++;
+                    }
+                }
+                return $text;
+            }
+        ],
+        'difficulties',
+        'tomorrow',
+        ['class' => 'yii\grid\ActionColumn'],
+    ],
+]);
+die();
+