Serving large files (>1GB) via PHP on Plesk / Nginx / Apache - php

Question

I'm trying to serve arbitrary large files via PHP.
Since I need to check permissions first, i cannot let the Web server handle the file downloads directly.
The server is running Plesk 17.0. As far as I know, Plesk uses Nginx as a proxy for Apache by default. But this can be turned off, so Apache serves everything directly.
My Problem:
On downloads I get a Network error at a few MB after 1GB if I use the default configuration (which is with Nginx running).
I've read many suggestions on how to handle large file uploads in PHP. Currently I'm using essentially this:
if ( isset( $_SERVER['MOD_X_SENDFILE_ENABLED'] ) &&
$_SERVER['MOD_X_SENDFILE_ENABLED'] == 1 ) {
header( "X-Sendfile: " . $this->options['base_path'] . $file );
} else {
readfile( $file );
}
As you see, I've tried offloading the file download to Apache. It also works fine, if I turn off Nginx.
With Nginx turned on, the max_execution_time in PHP marks a limit at which Nginx produces a network error. Though it serves at least 1GB. To me it seems, that there is some kind of block size limit between Apache and Nginx, which is set to 1GB. But I could not find such an option. For example setting max_execution_time to 5 sec, still delivers 1GB, even if the Download takes 10 Minutes.
This error is logged in the proxy_error_log when 1GB is served and the max_execution_time is passed:
[error] 3524#0: *796853 upstream prematurely closed connection while reading upstream
With Apache serving directly and mod_xsendfile active the max_execution_time does not matter. Using PHP readfile the max_execution_time matters. This also makes sense to me.
But according to the Plesk documentation it is beneficial to use Nginx for the serving.
So I'm looking to a way to keep Nginx and Apache running and not being limited by the max_execution_time when serving multiple GB files.