未定义的索引HTTP_HOST错误codeigniter

I don't know what I did. As long as I remember, I never touched any http host configurations. Suddenly, my code-igniter application is not working. I have 2 points of view, front end and back end.

The system front end works perfectly fine, but I cannot access the system in back end.

I checked my php_error_log file, and I got this message :

PHP Notice: Undefined index: HTTP_HOST in D:\Xampp\htdocs\index.php on line 7

I opened htdocs/index.php , this is the file :

I don't know what to do, please help me. :(