PHP connection to MongoDB cluster

Hello, I’ve difficulties with connection to MongoDB cluster on DigitalOcean by PHP. As we know, we have to use SSL, but I don’t know how. My code is as follows:

<?php
require_once __DIR__ . "/vendor/autoload.php";
$db = new MongoDB\Client('mongodb://[myapp].mongo.ondigitalocean.com', array(
    'username' => 'doadmin',
    'password' => '[password]',
    'db'       => '[database]',
	'authSource' => 'admin',
	'replicaSet' => '[replica]',
	'tls' => 'true',
	'tlsCAFile' => '/usr/share/ca-certificates/app/ca-certificate.pem'
));

$db = $conn->[database];
$collection = $db->[collection];
?>

I am getting an error: ‘Uncaught MongoDB\Driver\Exception\ConnectionTimeoutException: No suitable servers found’.

Do you know how to connect properly in this case? Thank you in advance.