[php-src] Issue #12385: `flush` with fastcgi does not force headers to be send

Issue: https://github.com/php/php-src/issues/12385 Author: mvorisek ### Description The following code: ```php <?php while (ob_get_level() !== 0) { ob_end_flush(); } flush(); var_dump(headers_sent()); var_dump(ob_get_level()); ``` Resulted in this output: ``` bool(false) int(0) ``` But I expected this output instead: ``` bool(true) int(0) ``` This issue is reproducible with fastcgi on Windows together with nginx. When I output/echo any data, the headers are always sent (expected). When I use the php internal webserver instead, the flush is enough to force the headers to be send without any data/body (expected). ### PHP Version 8.1 ### Operating System Windows 10