is codeigniter 3 safe enough to use?

PHP Framework can solve basic problem and security.
But it still require security skill of programmer on every it.

CI are the same, it solve basic web security (or reduce cause of any problem in the future).

If you care about security please read about web security and adapt it in every line of your code (not the best solution because it spend most time to it but it can make sure you did not made some mistake).