Bays - Easy "widgets" for your views

Is it possible to use Bay within CodeIgniter2 environment? What modifications should I make in Bay's source code?

Since it's loaded via Composer, you'd first have to get that working with your CI2 environment. In the past, I did this by copying the relevant code from CI3, which is in /system/core/CodeIgniter.php.

After that, it looks like it should work, but you may need to implement a CacheInterface and LibraryFinderInterface for CI2. You could probably use the supplied CI3 implementations as a starting point for CI2 (in fact, they may work for CI2 without modification, but I'm not 100% sure on that).

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide