`http_response_code` must emit a warning if headers were already flushed

[mvorisek]

Description

The following code:

<?php

echo 'x';
ob_end_flush();
flush();
// header('test: test');
http_response_code(301);

Resulted in this output:

x

But I expected this output instead:

x
Warning:  Cannot modify response code - headers already sent by (output started at ....php:12) in ...

I would expect about the same warning as header() emit. In the repro above, I have verified header(string) emits a warning, but http_response_code(number) currently does not.

PHP Version

any

Operating System

any