Under PHP VERSION 7.1.0
As i know that
Notice: Undefined variable: http_response_header in C:\xampp\htdocs\myfiles\Research\index.php on line 2
Even though you asked for not redirecting you back to php.net, I will still do that:
When using the HTTP wrapper, $http_response_header will be populated with the HTTP response headers.
This means, you have to use a Wrapper in your php script. Calling your document triggers a GET to your Apache but not a wrapper in your script.