php mongodb connect with x509

I have both legacy mongo php 1.6.10 driver and supported 1.2.5 mongodb php driver installed. php version is 5.6.29 on Debian 8.

both legacy driver and supported driver can connect using basic credentials.

Only the legacy driver can connect using the x509 certificate.

The supported driver causes the below exception when trying to do a simple findOne on a collection.

PHP Fatal error:  Uncaught exception 'MongoDB\Driver\Exception\RuntimeException' with message 'SCRAM Failure: invalid salt length of 0 in sasl step2'

I am using the Mongodb Client library for the mongodb driver http://php.net/manual/en/set.mongodb.php

Here is paraphrased code I am using

<?php
$server = 'mongodb://uat-a:27017,uat-b:27017,uat-c:27017';
$options = [
    'replicaSet' => 'rs-uat',
    'username' => 'CN=my-user,OU=user,O=NA,L=Place,ST=State,C=GB',
    'authMechanism' => 'MONGODB-X509',
    'authSource' => '$external',
    'ssl' => true,
    'connect' => true,
];
$driverOptions = [
    'context' => stream_context_create(
        [
            'ssl' => [
                'local_cert' => '/etc/local-cert.pem',
                'cafile' => '/etc/cafile.pem',
            ],
        ]
    ),
];
$database = 'uatdata';

$client = new MongoDB\Client($server, $options, $driverOptions);
$db = $client->selectDatabase($database);

$doc = $db->selectCollection('errors')->findOne([], ['projection' => ['timestamp' => 1, 'uri' => 1]]);