Session: Creates new session on every page refresh

First of all, thanks for your helps. I solved the problem.

As i said before; I am using PHP 7.1.2 and IIS 7 on Windows Server 2008 R2. Also, for CodeIgniter my version is 3.1.

I updated the CI with latest version and problem has solved. If someone gets same problem, it might be about PHP and CI version conflict. I realized that there are some changes in new PHP version in that subject.