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

使用提供的 PHP CLI 命令重现该行为，将空输出缓冲区与包含输出的缓冲区进行比较，并检查是否仍然可以发送 headers。然后检查 ob_end_flush() 路径和 zlib 输出处理程序；当空缓冲区不会隐式执行 flush，而非空输出仍保持预期行为时，即表示完成。