Where to put plain object classes?

But when the target class is static... What then?

I have another way by using an autoloader, based on the function spl_autoload_register(). Have a look at this project as an example: https://github.com/ivantcholakov/codeigniter-utf8

These classes I place within a separate folder application/classes/. Then, the autoloader may be declared and registered within a 'pre_system' hook: https://github.com/ivantcholakov/codeign...toload.php . This is just a non-standard example, you can tweak and extend the autoloader.

If the third-party class is available as a Composer package - that would be the preferred choice, of course.