How to add HEADER in apache_request_headers in PHP - php

Question

I want to add a header, to request headers, dynamically on the server side.
I am using slim 2 framework which supports middleware.
Here is my usecase:
Client initiates request to url "https://somedomain.com/login" with some Request Headers.
I have middleware say authenticate. Which should add say "UserAddress" to Request Header.
My callback function login() is called. And I need to access "UserAddress" from header.
So following is the code for this route:
function login() {
//Login related stuff
$allHeaders = apache_request_headers();
//Perform some operation on UserAdress from $allHeaders
}
function authenticate(\Slim\Route $route) {
//Perform authentication here
//I am using SUPERFICIAL method `set_apache_request_headers` as reference.
//Here I need to know how I can add new header to REQUEST HEADER
set_apache_request_headers('UserAdress', 'New York');
//Here is what I tried, which did not work when I called apache_request_headers()
//$_SERVER["UserAdress"] = "New York";
}
$app = getSlimInstance();
$app->post('/login', 'authenticate', login);
I tried using $_SERVER, but when I call apache_request_headers(), my header does not show up.
Side Note:
I am using "UserAddress" as my header for reference purpose. Actually I am using different name.
Also I know you guys will say pass that via request body. But due to legacy code I need this in request header.
I just need to know how can I modify the Request Header