src/collections/Message.php
public function renderHeader()
    {
        if ($this->header !== false || !empty($this->header)) {
            $content = ArrayHelper::getValue($this->header, 'label', '');
            $options = ArrayHelper::getValue($this->header, 'options', ['class' => 'header']);
            $header = Ui::tag('div', $content, $options);
            $lines[] = Ui::beginTag('div', ['class' => 'content']);
            $lines[] = $header;
            return implode("\n", $lines);
        } else {
            return null;
        }
    }
public function renderHeader()
    {
        if ($this->header !== false || !empty($this->header)) {
            $content = ArrayHelper::getValue($this->header, 'label', '');
            $options = ArrayHelper::getValue($this->header, 'options', ['class' => 'header']);
            $header = Ui::tag('div', $content, $options);
            $lines[] = Ui::beginTag('div', ['class' => 'content']);
            $lines[] = $header;
            return implode("\n", $lines);
        } else {
            return null;
        }
    }
Renders the header (if any)

@return null|string