as/oArray extraFields -> to not get it

I do have this code to add an extra field to an array: all the names of users. It should work with toArray. But toArray is not available, only asArray. But asArray returns an empty array. So I do not get the concept of extraFields, I think. What is the differecent between toArray and asArray? Why are they different?

    public function actionIndex()


	$spellen = Spelrondeactievespelers::find()

		->where(['tbl_spelrondeactievespelers.user_id' =>  Yii::$app->user->id])	

		->select(['tbl_spelrondeactievespelers.spelrondeactievespelers_id', 'tbl_spelrondeactievespelers.spelronde_id','tbl_spelvorm.spelvormnaam'])


		->asArray([], ['namen'])


        return $this->render('index', [

            'spellen' => $spellen,



public function extraFields()


	return [

	    'namen'=>function() {

		$spelers = Spelrondeactievespelers::find()

			->where(['spelronde_id' => $spelronde])	





		$namen = [];

		foreach($spelers as $speler) {

			$namen[]= $speler->name;


		return implode(', ', $namen);