Description:
------------
Not sure if other SAPIs are affected too, but at least I could not reproduce this with PHP's builtin server.

If sending HTTP headers with the header() method, these headers are no longer sent if the HTTP response body is empty. This can be worked around by flush()ing manually.

PHP 7.0.25

Test script:
---------------
header('X-Foo: bar');
// flush();
// echo "foo";
exit;

Expected result:
----------------
HTTP/1.1 200 OK
Date: Thu, 01 Mar 2018 15:24:28 GMT
Server: Apache/2.4.18 (Ubuntu)
X-Foo: bar
Vary: Accept-Encoding
X-Mod-Pagespeed: 1.13.35.2-0
Cache-Control: max-age=0, no-cache
Transfer-Encoding: chunked
Content-Type: text/html; charset=UTF-8


Actual result:
--------------
HTTP/1.1 200 OK
Date: Thu, 01 Mar 2018 15:24:12 GMT
Server: Apache/2.4.18 (Ubuntu)
Cache-Control: max-age=0, no-cache
Content-Length: 0
Content-Type: text/html; charset=UTF-8