I really want just the most simple system to work. presumably, one php page to put up the login choice to the user (perhaps but not necessarily including sending-an-email as authentication choice) and one php page each to handle error and success. success means a verified email address. in the old and google-only page, this meant a gidtoken.
request:
where ... is presumably a list of choices (google, github, facebook, etc.) with things like appsecrets.
success:
<?php
require_once('firebase.php');
$goodemail = decodetoken();
?>