is codeigniter 2.2.6 is compatible with php 7 ?

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.

Codeigniter 3 has been compatible with PHP 7 for months. What you are saying is completely wrong. And no one should be waiting for CI 4. CI 4 is going to be a big rewrite of your application.

A great strategy would be to move your application to PHP 7 as soon as possible so its compatible. Then get more experience learning and working with PHP 7. As a bonus your application will run much faster on PHP 7. Then when you are ready to upgrade to CI 4 it will be much easier.