https://site.com/home/index.php?email=first.last@thomsonreuters.com&firstname=First&lastname=Last&marketgroup=43003&uid=4334525&time=2014:07:24:19:14:57&digest=1758b84916f36245cad17fd325f8d3&returnurl=https://site.com/sandbox/subsite

define('SSO_REDIRECT', 'https://sso.com/login/sso/SSOService?app=' . SSO_APP_ID . '&returnurl=' . urlencode('https://site.com/sandbox/subsite'));

// Start authentication and authorization
if (!isset($_SESSION['USER_ID']) && !isset($_POST['digest'])) {
    // We've got no session and we've not returned from SSO, so let's redirect to SSO
    header('Location: ' . SSO_REDIRECT);
    die();
}