有没有办法用PHP确定上游Web服务器？

I'm developing a PHP web application that needs to send out a file in a server specific way, like so:

<?php
$server = get_upstream_web_server();

if ($server === 'nginx') {
  header('X-Accel-Redirect: smiley.png');
}
else if ($server === 'apache') {
  header('X-Sendfile: smiley.png');
}
else {
  echo file_get_contents('smiley.png');
}

Is there a way to get the name of the upstream server like the get_upstream_web_server() in the above example?

See $_SERVER['SERVER_SOFTWARE'].