session & multi subdomains

Hello,
i create a website with codegneiter and i have a subdomain builded by simple PHP,
and i have user login from main url or blog.. my problem when user connect from blog and go to main url (codegneiter) must relogin agian from site!!
how i can when user logged from blog and go to main site see his account not need relogin again?!
please explain me step by step because i'm ,new with codegneiter..

thanks