Gzip output handler - `ob_end_flush()` must not call `flush()`

Description

This issue is problematic only when zlib.output_compression is enabled.

Currently, when zlib.output_compression is enabled like:

php -S 127.0.0.1:81 -d zlib.output_compression=1 -t .

(but also tested if enabled when used /w Apache /wo mod_deflate)

and ob_end_flush() is called, no headers cannot be send anymore even if the nothing was output.

Currently, we had to use the following code

while (ob_get_level() > 0) {
    if (ob_get_length() === 0) {
        ob_end_clean();
    } else {
        ob_end_flush();
    }
}

to workaround.

When zlib.output_compression is NOT enabled, the following code:

while (ob_get_level() > 0) {
    ob_end_flush();
}

is working (allows to send additional headers).

Because of this inconsistency, I belive php gzip output handler behaviour should be fixed, when output buffer is empty, to not flush implicitly.

PHP Version

any

Operating System

any

Reproduce el comportamiento con el comando PHP CLI proporcionado, comparando un búfer de salida vacío con otro que contenga salida y comprobando si aún se pueden enviar headers. Después, inspecciona la ruta de ob_end_flush() y el gestor de salida de zlib; se considera terminado cuando un búfer vacío no hace flush implícitamente, mientras que la salida no vacía conserva su comportamiento esperado.