Get data from cookie on subdomain with Codeigniter

I don't understand you response. You said this

I created sample file on sub.mydomain.com/test.php where I am able to get data from session using php without codeigniter => so data are available there...But I need to get them with Codeigniter. Any ideas?

So if you could read the data with the sample file on your subdomain with native PHP, how is it you don't know how to assign it to a variable?