It does explain this:

"When using the HTTP wrapper, $http_response_header will be populated with the HTTP response headers."

It doesn't claim anywhere that $http_response_header is supposed to be a superglobal. What do you suggest the documentation should say?

It's true. The documentation doesn't say that $http_response_header is
superglobal. It doesn't say anything ;-)
I think it's normal to suposse that it's a superglobal. I thinked so.
A little note saying:

"It's  a local variable that is created in the same context you call
'file_get_contents' ..."

(using a correct english ... please ;-) can help to avoid errors.

Thanks!

The variable $http_response_header is an array which is created in the local 
scope.


<?php
function a()
	{
	echo 'In scope : Before : ', (isset($http_response_header) ? 'Exists' : 
'Does not exist'), PHP_EOL;
	file_get_contents('http://www.google.com');
	echo 'In scope : After : ', (isset($http_response_header) ? 'Exists' : 
'Does not exist'), PHP_EOL;
	}

echo 'Out of scope : Before : ', (isset($http_response_header) ? 'Exists' : 
'Does not exist'), PHP_EOL;
a();
echo 'Out of scope : After : ', (isset($http_response_header) ? 'Exists' : 'Does 
not exist'), PHP_EOL;
?>

outputs ...

Out of scope : Before call : Does not exist
In scope : Before call : Does not exist
In scope : After call : Exists
Out of scope : After call : Does not exist

Jejeje! It's too much!!! ;-)
For me, this part is enought:

"The variable $http_response_header is an array which is created in the local 
scope."

But the full version would help somebody ...