Accessing a custom header in the API - php

Question

I am sending a custom header to an API that I control using an AJAX call made via AngularJS.
Client (AngularJS)
var authorization_token = 'qwerty';
var custom_token_value = 'abc123';
$http.get('http://api.mywebsite.com/endpoint',{ headers:{'Auth':authorization_token,'Custom_Header':custom_token_value} }).then(function(res){
console.log(res.data);
});
Server (PHP 5.6)
header('Access-Control-Allow-Origin: *');
header('Access-Control-Allow-Methods: GET,POST,PUT,DELETE');
header("Access-Control-Allow-Headers: Origin, X-Requested-With, Content-Type, Accept, Auth, Custom_Header");
$headers = getallheaders();
print_r($headers);
Response
Array
(
[Accept-Language] => en-US,en;q=0.9
[Accept-Encoding] => gzip, deflate
[Referer] => http://localhost:8080/
[Auth] => qwerty
[User-Agent] => Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.119 Safari/537.36
[Origin] => http://localhost:8080
[Accept] => application/json, text/plain, */*
[Connection] => close
[Host] => api.mywebsite.com
)
Why am I not able to see Custom_Header?

Answer 1

For anyone having this issue, after some trial and error I've discovered that the reason Custom_Header was not showing up is because it contained an _ (underscore) character. Underscore characters apparently get dropped from header keys.
More info here: Why underscores are forbidden in HTTP header names