如何在yii2的下拉列表中显示名字和姓氏

use app\models\User;
use yii\helpers\ArrayHelper;

public function actionIndex()
{
    $users = User::find()->all();
    $userList = ArrayHelper::map($users, 'id', function($user) {
        return $user->first_name . ' ' . $user->last_name;
    });

    return $this->render('index', [
        'userList' => $userList,
    ]);
}

use yii\helpers\Html;

echo Html::dropDownList('user', null, $userList, ['prompt' => 'Select User']);