is codeigniter 2.2.6 is compatible with php 5.6.6 ?

i can confirm it works -- you might get warning messages about deprecated etc etc.

but note that - its actually pretty easy to upgrade to CI version 3.

I can't say that it is easy for big or even huge projects but highly recommended if you're going to support and customize it.
Besides warning messages about deprecated there may be warnings about pointers (if project was developed with php < 5.3).
At least all controllers have to be renamed to match their class name (case sensitive).

We support a big project on CI 1.7 with a lot of customs and it's not a trivial task to convert it to even CI 2.x. But works it with php 5.6.x.