Where to put plain object classes?

easiest way:

put it into the application/libraries folder
autoload this library in your application/config/autoload.php

then from anywhere in your controllers/models user: $this->library_name_here->some_function();

from other libraries in application/libraries or from helper files: $ci = get_instance(); $ci->library_name_here->some_function();

"I reject your reality and substitute my own" - Adam Savage, M5 Inc.