src/Widgets/Bootstrap/Card.php
protected function renderHeader() {
        if (!isset($this->header)) {
            return '';
        }
        if (isset($this->headerOptions)) {
            Html::addCssClass($this->headerOptions, ['widget' => 'card-header']);
            $tag = ArrayHelper::remove($this->headerOptions, 'tag', 'div');
            $header = Html::tag($tag, $this->header, $this->headerOptions);
        } else {
            $header = $this->header;
        }
        return $header;
    }
protected function renderHeader() {
        if (!isset($this->header)) {
            return '';
        }
        if (isset($this->headerOptions)) {
            Html::addCssClass($this->headerOptions, ['widget' => 'card-header']);
            $tag = ArrayHelper::remove($this->headerOptions, 'tag', 'div');
            $header = Html::tag($tag, $this->header, $this->headerOptions);
        } else {
            $header = $this->header;
        }
        return $header;
    }
Render card header
@return string