index.php

CodeIgniter's index.php is crucial, because all server requests are 'captured' by it, and it's the starting point for all controllers.
On the other hand, XAMPP's index.php is also very important.
What you need to do is create a folder inside the htdocs folder. Give the folder a name that corresponds with your project.
Put all CodeIgniter files and folders in your project's folder.

Let's say your project is called "myproject", then you call your project's home page with this url:
localhost/myproject