I removed the following line:
<?php require_once('../webassist/mysqli/authentication.php'); ?>
...and it works in localhost. So I'm guessing that I need to update something associated with the Login User behaviour, except I have some custom coding in this behaviour, to add additional security, so what do I need to update and how can I do that without disrupting my hand coded Login User behaviour?
EDIT: Removed the entire behaviour, added the standard behaviour, then replaced the behaviour code with my custom coded behaviour. Sorted!
Which file updated though? I'll need to update all occurences of this file on all sites that use it, right?