Custom HTTP adapters
By default, the HTTP(S) XRD files are loaded by XML_XRD internally using simplexml_load_file, which emits PHP Warnings when the files are not found or other HTTP errors occur.
To work around that limitation, or to set some custom HTTP request headers, you may use an own HTTP adapter that's used to fetch the files:
<?php
require_once 'HTTP/Request2.php';
require_once 'Net/WebFinger.php';
$req = new HTTP_Request2();
$req->setConfig('follow_redirects', true);//needed for full compatibility
$req->setHeader('User-Agent', 'MyApp 1.42');
$wf = new Net_WebFinger();
$wf->setHttpClient($req);
$react = $wf->finger('foo@example.org');
?>