code: public function beginField($model, $attribute, $options = [])
      {
          $field = $this->field($model, $attribute, $options);
          $this->_fields[] = $field;
          return $field->begin();
      }
docs: Begins a form field.
  This method will create a new form field and returns its opening tag.
  You should call [[endField()]] afterwards.
  @param Model $model the data model.
  @param string $attribute the attribute name or expression. See [[Html::getAttributeName()]] for the format
  about attribute expression.
  @param array $options the additional configurations for the field object.
  @return string the opening tag.
  @see endField()
  @see field()