If your existing site does not use authentication (user accounts) and sessions, the simple answer is to provide links in the navigation system of the existing site to the CodeIgniter application and links on the navigation system of the CodeIgniter application to the existing site. This is usually done via a menu system. If the existing site uses authentication and sessions, you either need to adapt the existing site to use CodeIgniter code for handling authentication and sessions or vice versa.
However, you would be better off converting the existing site to CodeIgniter’s MVC architecture if nothing else as a learning experience.
When I trying to use codeigniter the index page moves to default controller. I cant use as index existing index.php file.
I don’t know how to include codeigniter files and use codeigniter for new pages only with the current index.php file.
unfortunately I can’t convert existing files into codeigniter MVC because almost all the pages have already google page rank and if I change links to pages it will be bad for SEO