is codeigniter 2.2.6 is compatible with php 7 ?

I can't say about CI 2.x, but I tried with CI 3.x.
There were a lot of warnings and errors so I back to php 5.6.x.

I think we should wait for CI 4.

Sure, you can't get yourself to make a trivial update, but somehow waiting for a version with a ton of BC breaks makes sense ... how?!

Not for that project I told there.
Here I told about using php 7 with CI. It'll be better to wait for CI 4.

Migration from CI 2 to CI 3 is trivial but do not forget about projects specifics. Sometime is better to not to migrate. It would take much time.