gnu-social/DOCUMENTATION/SYSTEM_ADMINISTRATORS/socialfy-another-domain/dot-well-known/webfinger/index.php

<?php

// FIXME: REPLACE          \/ here
define('MY_GNUSOCIAL', 'https://www.example.org/gnusocial/index.php');

/**
 * This is a general solution for when you can't have your GNU social instance in the domain root and for when you want to
 * socialfy from another domain.
 */

// From https://www.php.net/manual/en/function.getallheaders.php#84262 (joyview at gmail dot com)
if (!function_exists('getallheaders')) {
    function getallheaders()
    {
        $headers = [];
        foreach ($_SERVER as $name => $value) {
            if (substr($name, 0, 5) == 'HTTP_') {
                $headers[str_replace(' ', '-', ucwords(strtolower(str_replace('_', ' ', substr($name, 5)))))] = $value;
            }
        }
        return $headers;
    }
}
$ch = curl_init();
curl_setopt($ch, CURLOPT_HTTPHEADER, getallheaders());
curl_setopt($ch, CURLOPT_URL, MY_GNUSOCIAL . str_replace('webfinger/', 'webfinger', $_SERVER['REQUEST_URI']));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HEADER, true);
$response = curl_exec($ch);
$header_size = curl_getinfo($ch, CURLINFO_HEADER_SIZE);
$body = substr($response, $header_size);
// From https://stackoverflow.com/a/10590242 (c.hill)
function get_headers_from_curl_response($response)
{
    $headers = array();

    $header_text = substr($response, 0, strpos($response, "\r\n\r\n"));

    foreach (explode("\r\n", $header_text) as $i => $line) {
        if ($i === 0) {
            $headers['http_code'] = $line;
        } else {
            list($key, $value) = explode(': ', $line);

            $headers[$key] = $value;
        }
    }

    return $headers;
}
$headers = get_headers_from_curl_response($response);
foreach ($headers as $name => $value) {
    header("{$name}: $value");
}
echo $body;
curl_close($ch);