This is yet another RSS / Atom feed generator for PHP 7 and later. It works great in combination with Flow but should also be fine as a standalone library.
This package is composer enabled.
<?php
$feed = new \RobertLemke\Rss\Feed();
$channel = new \RobertLemke\Rss\Channel();
$channel
  ->setTitle('All about Neos Flow')
  ->setDescription($channelDescription)
  ->setFeedUri($feedUri)
  ->setWebsiteUri($websiteUri)
  ->setLanguage('en-US');
$item = new \RobertLemke\Rss\Item();
$item
  ->setTitle('My first blog post')
  ->setGuid($someUniqueIdentifier)
  ->setPublicationDate(new DateTime())
  ->setContent($blogPostContent);
$channel->addItem($item);
$feed->addChannel($channel);
echo $feed->render();