- re-evaluate the use of trigger_error() or use a custom error handler,
  because right now the code exists as soon as trigger_error() gets called and
  any following code is ignored.

- write better XSL stylesheets? I am not an XSL expert
- evaluate the use of the <ttl/> RSS element
- show cards directly in RSS items for twitter.com
- show direct links for videos in the Instagram feed
- check the encoding of the tweets when UTF is used,
  maybe solvable with mb_convert_encoding()?
  See http://php.net/manual/en/domdocument.loadhtml.php

- The dependencies on the symphony components in composer.json could be more
  relaxed like ">=2.7.0", but for now sticking to "2.7.*" is good enough.

- Add support for instagram tags
