[php-src] Issue #13164: flush() doesn't actually flush

Issue: https://github.com/php/php-src/issues/13164 Author: kkmuffme ### Description Followup to https://github.com/php/php-src/issues/12385 - since this added a "feature/fix?" that didn't actually fix the issue* (*the original issue seems to work correctly in PHP-FPM 7.4/8/8.1/8.2 too for me without the fix - @nielsdos were you actually able to reproduce the reported issue?) It just looked like it did fix the issue, because var_dump was used to output text. When there are only headers (which was the original intention - since we want to send the headers asap and not wait for the rest of the body), it doesn't work: ```php <?php // header( 'X-Abc: Hello'); - actually setting a header makes no difference $first = headers_sent(); while (ob_get_level() !== 0) { ob_end_flush(); } $second = headers_sent(); flush(); $third = headers_sent(); var_dump($first); $fourth = headers_sent(); var_dump($second); var_dump($third); var_dump($fourth); ``` You get 3x false and the headers are only actually sent after the first var_dump call (therefore $fourth is true) Bug: Currently flush() (and/or ob_implicit_flush being enabled) will only trigger if there is any non-empty output. (e.g. adding echo ''; before the flush() won't do anything, however echo 'a'; will make it flush). I assume this is also the reason that ob_gzhandler (and/or zlib.output_compression) starts to output the initial gzip bytes immediately on first callback, as this non-empty return value will force flush the headers (and therefore break all responses with no body, e.g. HTTP 304,... won't work when using zlib.output_compression) so that later code cannot modify the Content-Encoding,... headers. ### PHP Version 8.3.1 ### Operating System All