Aerys\Response::addHeader PHP Method

    public function addHeader(string $field, string $value) : Response;

 private function writeResponse(Request $request, Response $response, ApiResponse $result)
 {
     $response->setStatus($result->getStatus());
     $response->setHeader("content-type", "application/json");
     foreach ($result->getLinks() as $rel => $params) {
         $uri = strtok($request->getUri(), "?");
         $uri .= "?" . http_build_query($params);
         $elements[] = "<{$uri}>; rel=\"{$rel}\"";
     }
     if (isset($elements)) {
         $response->addHeader("link", implode(", ", $elements));
     }
     $response->send(json_encode($result->getData(), JSON_PRETTY_PRINT));
 }