php -S localhost:8000
If I understand properly,
phpdbg_break is a function provided by the interpreter, and not by any extension. Instead of using the built-in PHP server, you should use the phpdbg server, and simulate a web request. See http://phpdbg.com/docs/mocking-webserver for information on how to mock the request and http://phpdbg.com/docs/simples to know how to run the debugger.
To make a request to the specific URI, I think you need to set
$_SERVER['REQUEST_URI'] and optionally
$_SERVER['QUERY_STRING'] to point to the URL you want to test. URI will be something like '/path/to/file' and the querystring would be everything between the ? and the # in the URLs (ie ?page=2)
Thanks for pointing me to phpdbg, I didn't know that tool and it seems to be a very good one; I'll be testing it in the next days.