Remove 'heredoc'

[JakeQZ]

'heredoc' is a hangover from when PHP was just a simple scripting language.

Now it is a fully-fledged OO language, the syntax breaks readability in classes, etc., and should not be used (IOO).

• Replace all 'heredoc'
• Add code sniffer rule (or suchlike) to not allow it

[oliverklee]

I have checked the PHP_CodeSniffer and PHP-CS-Fixer rules and was unable to find any rules for this. Maybe we need to write a small Composer script to grep for the <<< heredoc marker.