OKyJIucT

public function actionCreate($event_id = null)
{
	if($event_id) {
		$model = new RegBattle(['event_id'=>$event_id]) //event_id получаем из get запроса
	} else {
		// ... другое
	}
        

}

['param', 'integer', 'min' => '1', 'max' => '9', 'when' => function($m) {
    return $m->param == 'sku_max'; 
}],

$form->field($form, 'bids[' . $shop->id . ']')->textInput(['pattern' => '[1-9]\d*'])->label(false)

['zip', 'exists', 'targetClass' => ZipKz::className(), 'targetAttribute' => ['zip_old', 'zip_new'], 'when' => function($model){
  return $model->geo == 'KZ';
}],