Hi there!
As per my understanding, please feel free to correct me - Your existing users are stored in CodeIgniter, and you want users to authenticate into Moodle using the same credentials.
I had a similar use case with Laravel, where I wanted the users to login into Moodle using the Laravel credentials.
miniOrange offers a perfect SSO solution for Moodle LMS according to your requirement.
As per my understanding, please feel free to correct me - Your existing users are stored in CodeIgniter, and you want users to authenticate into Moodle using the same credentials.
I had a similar use case with Laravel, where I wanted the users to login into Moodle using the Laravel credentials.
miniOrange offers a perfect SSO solution for Moodle LMS according to your requirement.
You could simply connect miniOrange IDP with your external database, and configure CodeIgniter and Moodle as Service Providers, enabling you to login into Moodle with the credentials of CodeIgniter and setting up SSO for both the applications.