Memory problems with php://input - php

Question

I have an API endpoint that can receive a POST JSON/XML body (or even raw binary data) inside the body content as payload that should be written immediately to a file on the filesystem.
For backwards compatibility reasons, it cannot be a multipart/form-data.
It works with no problems for body content up to a certain size (around 2.3GB with a 8GB script memory limit).
I've tried all of the followings:
both with and without setting the buffers' sizes
$filename = '/tmp/test_big_file.bin';
$input = fopen('php://input', 'rb');
$output = fopen($filename, 'wb');
stream_set_read_buffer($input, 4096);
stream_set_write_buffer($output, 4096);
stream_copy_to_stream($input, $output);
fclose($input);
fclose($output);
and
$filename = '/tmp/test_big_file.bin';
file_put_contents($filename, file_get_contents('php://input'));
and
$filename = '/tmp/test_big_file.bin';
$input = fopen('php://input', 'rb');
$output = fopen($filename, 'wb');
while (!feof($input)) {
fwrite($output, fread($input, 8192), 8192);
}
fclose($input);
fclose($output);
Unfortunately, none of them works. At one point, I get always the same error:
PHP Fatal error: Allowed memory size of 8589934592 bytes exhausted (tried to allocate 2475803056 bytes) in Unknown on line 0
Also unsetting enable_post_data_reading makes no difference and all the php.ini post/memory/whatever sizes are set to 8GB.
I'm using php-fpm.
Looking what's happening at the memory with free -mt, I can see that the memory used increases slowly at the beginning, going faster after a while, up to a point that no more free memory is left, so the error.
On the temp directory, the file is not directly stream-copied, but instead it is written on a temporary file named php7NARsX or other random strings which is not deleted after the script crashes, so that at the following free -mt check, the available memory is 2.3GB less.
Now my questions:
Why the stream is not copied directly from php://input to the output instead of loading it into memory? (also using php://temp as output stream leads to the same error)
Why is PHP using so much memory? I'm sending a 3GB payload, so why it needs more than 8GB?
Of course, any working solution will be much appreciated. Thank You!