framework/widgets/ActiveField.php
public function begin()
    {
        if ($this->form->enableClientScript) {
            $clientOptions = $this->getClientOptions();
            if (!empty($clientOptions)) {
                $this->form->attributes[] = $clientOptions;
            }
        }

        $inputID = $this->getInputId();
        $attribute = Html::getAttributeName($this->attribute);
        $options = $this->options;
        $class = isset($options['class']) ? (array) $options['class'] : [];
        $class[] = "field-$inputID";
        if ($this->model->isAttributeRequired($attribute)) {
            $class[] = $this->form->requiredCssClass;
        }
        $options['class'] = implode(' ', $class);
        if ($this->form->validationStateOn === ActiveForm::VALIDATION_STATE_ON_CONTAINER) {
            $this->addErrorClassIfNeeded($options);
        }
        $tag = ArrayHelper::remove($options, 'tag', 'div');

        return Html::beginTag($tag, $options);
    }
public function begin()
    {
        if ($this->form->enableClientScript) {
            $clientOptions = $this->getClientOptions();
            if (!empty($clientOptions)) {
                $this->form->attributes[] = $clientOptions;
            }
        }

        $inputID = $this->getInputId();
        $attribute = Html::getAttributeName($this->attribute);
        $options = $this->options;
        $class = isset($options['class']) ? (array) $options['class'] : [];
        $class[] = "field-$inputID";
        if ($this->model->isAttributeRequired($attribute)) {
            $class[] = $this->form->requiredCssClass;
        }
        $options['class'] = implode(' ', $class);
        if ($this->form->validationStateOn === ActiveForm::VALIDATION_STATE_ON_CONTAINER) {
            $this->addErrorClassIfNeeded($options);
        }
        $tag = ArrayHelper::remove($options, 'tag', 'div');

        return Html::beginTag($tag, $options);
    }
Renders the opening tag of the field container.
@return string the rendering result.