CodeIgniter 3 PHP CodeSniffer

I'm not aware of any significant changes to the style guide for CI3 vs. CI2, except that you need to check for ucfirst file/class names on most of the classes.

Which is a functional requirement and not just a styleguide change.

You don't need to follow our styleguide in order to run CI.

I agree, especially since I only follow CI's style guide when I'm writing something I intend to submit to CI. However, it's much easier to do that if I have CodeSniffer configured properly, since my editor is configured to run it every time I save a file.

Although it is a functional requirement rather than a style guide change, CodeSniffer can be configured to check the file/class name to remind you of the functional requirement, so you don't go through the additional steps of attempting to load the page in your dev environment before discovering that you missed the shift key when you typed the file/class name.

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide