Q:

cross subdomain login with cookies

I have some sites in subdomains which are made in wordpress and some are made in codeigniter framework. System is already made and they have their own database. I have to created a common login system for all subdomains.
Do anyone have any idea how to do it?

A:

You can save login information into cookies and share it between subdomains.
See Cookies and subdomains:
Cookies and subdomains

