yii\web\XmlResponseFormatter::format PHP Method

    public function format($response)
    {
        $charset = $this->encoding === null ? $response->charset : $this->encoding;
        if (stripos($this->contentType, 'charset') === false) {
            $this->contentType .= '; charset=' . $charset;
        }
        $response->getHeaders()->set('Content-Type', $this->contentType);
        if ($response->data !== null) {
            $dom = new DOMDocument($this->version, $charset);
            $root = new DOMElement($this->rootTag);
            $dom->appendChild($root);
            $this->buildXml($root, $response->data);
            $response->content = $dom->saveXML();
        }
    }

 /**
  * @param mixed  $data the data to be formatted
  * @param string $xml  the expected XML body
  * @dataProvider formatObjectDataProvider
  */
 public function testFormatObjects($data, $xml)
 {
     $head = "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n";
     $this->response->data = $data;
     $this->formatter->format($this->response);
     $this->assertEquals($head . $xml, $this->response->content);
 }