I had some very simple code on a website to handle logins on the root of the site using ion_auth:
if(!$this->ion_auth->logged_in())
{
redirect("/login");
}
That page posts to /auth/login, which is handled by ion_auth. If the login is successful, the user is redirected to the root of the site. Otherwise, they go back to /login to try again. I initially thought I'd forgotten the password, but with a series of var_dump() calls, I found that the login is successful, but the call to $this->ion_auth->logged_in() returns false anyway.
All of this is after the host upgraded to PHP 7 - the code worked before. I've tried upgrading ion_auth, but it didn't help.
There were session related problems in early CI 3.x version, documented here and here
several session related issues were resolved in the latest versions, see the change logs
solution: update to CI 3.1.7. (latest version as writing)