Warning on using PHP7 and Redis as Session Driver

Recently I uploaded my PHP version to PHP7. It was a hard job to upload all packages like Redis, PDO and memcached.
After successfully deploying, sometimes the app reports a fatal PHP error like the following(All of the following files are based on Codeigniter 3.0.2) :

Code:

Message:  session_regenerate_id(): Failed to create(read) session ID: user (path: )
Filename: Session/Session.php
Line Number: 625

I found a bug report about this error in https://bugs.php.net/bug.php?id=71187, in which the last reply gave the reason. PHP 7 and above require the user handler to return a 'string' for success. I don't understand what does that mean but official Redis repo has not fixed it.

Anyone who are using Redis as a session driver under PHP7 should notice this issue.

Update your CI version as well.