设置Content-Length时，zlib.output_compression不起作用

I have trouble compressing PHP output when the Content-Length header is set. Following this very similar question of mine, it seems I've narrowed down the issue, which happens even without defining output compression in Apache's .htaccess.

So I'm using Apache 2.4.18, .htaccess is empty, AddOutputFilterByType DEFLATE text/html text/plain text/xml is commented out in Apache configuration, because I want the compression to happen in PHP.

My minimal PHP code:

<?php

ini_set("zlib.output_compression", "On");

$output = "";

for ($i = 0; $i < 4000; $i++) {
    $output .= "Foobar ";
}

// header("Content-Length: " . strlen($output));
header("Content-Type: text/html");

echo $output;

exit;

It works fine as is, the output is compressed (it's not the case if I omit the ini_set call, as expected).

Now if I uncomment the Content-Length header line, suddenly the output is not compressed anymore.

EDIT: In my case, the problem is solved, see my comments below.