// code snippet after <form> is submitted

$feedURL = $_POST['feedurl']
$feed = new SimplePie($feedURL);
$feed->feed_url($url);
$feed->cache_max_minutes(30);
$feed->strip_ads(true);
$feed->replace_headers(true);
$feed->init();

if( what check do I put in here? I suppose it has to do with init()??)
  echo 'success!';
}else{
  echo 'invalid file. you need to submit a feed. try again.';
}