class Albums extends \yii\db\ActiveRecord
{

    public $countPhotos;

    ...

    public function attributeLabels()
    {
        return [
            'id' => 'ID',
            'name' => 'Name',
            'about' => 'About',
            'photographer' => 'Photographer',
            'email' => 'Email',
            'phone' => 'Phone',
            'created' => 'Created',
            'edited' => 'Edited',
            'countPhotos' => 'Count Photos',
        ];
    }

    public function getCountPhotos()
    {
        $_cnt = (new \yii\db\Query())
        ->select(['COUNT(id)'])
        ->from('photos')
        ->where(['album' => '1']);
        return $this->$countPhotos = $_cnt;
    }
}

<?php

use yii\helpers\Html;
use yii\widgets\DetailView;

/* @var $this yii\web\View */
/* @var $model app\models\Albums */

$this->title = $model->name;
$this->params['breadcrumbs'][] = ['label' => 'Albums', 'url' => ['index']];
$this->params['breadcrumbs'][] = $this->title;
?>
<div class="albums-view">

    <h1><?= Html::encode($this->title) ?></h1>

    <p>
        <?= Html::a('Update', ['update', 'id' => $model->id], ['class' => 'btn btn-primary']) ?>
        <?= Html::a('Delete', ['delete', 'id' => $model->id], [
            'class' => 'btn btn-danger',
            'data' => [
                'confirm' => 'Are you sure you want to delete this album?',
                'method' => 'post',
            ],
        ]) ?>
    </p>

    <?=
        $model->getCountPhotos();
        DetailView::widget([
        'model' => $model,
        'attributes' => [
            // 'id',
            'name',
            'about:ntext',
            'photographer',
            'email:email',
            'phone:ntext',
            'created',
            'edited',
            'countPhotos',
        ],
    ]) ?>

</div>

    <?=
        DetailView::widget([
        'model' => $model,
        'attributes' => [
            // 'id',
            'name',
            'about:ntext',
            'photographer',
            'email:email',
            'phone:ntext',
            'created',
            'edited',
            [
                'label' => 'countPhotos',
                'value' => $model->getCountPhotos(),
            ]

        ],
    ]) ?>