For example, click on a PHP function on this page:
https://secure.phabricator.com/diffusion/PHU/browse/master/src/xsprintf/queryfx.php
You will get:
[Rendering Exception/Exception] Refusing to redirect to local resource "http://www.php.net/function.whatevs". The URI has a domain, but the redirect is not marked external. Mark redirects as external to allow redirection off the local domain.