code: public function offsetGet($offset)
      {
          if ($offset === 'headers') {
              $headers = [
                  'HTTP/' . $this->getResponse()->getProtocolVersion() . ' ' . $this->getResponse()->getStatusCode() . ' ' . $this->getResponse()->getReasonPhrase()
              ];
              foreach ($this->getResponse()->getHeaders() as $header => $values) {
                  foreach ($values as $value) {
                      $headers[] = $header . ': ' . $value;
                  }
              }
  
              return $headers;
          }
      }
docs: Offset to retrieve.
  @link http://php.net/manual/en/arrayaccess.offsetget.php
  @param mixed $offset <p>
  The offset to retrieve.
  </p>
  @return mixed Can return all value types.
  @since 5.0.0