Unlike {@see \Google\Cloud\PubSub\PubSubClient::topic()}, this method will send an API call to
create the topic. If the topic already exists, an exception will be
thrown. When in doubt, use {@see \Google\Cloud\PubSub\PubSubClient::topic()}.
Example:
$topic = $pubsub->createTopic('my-new-topic');
echo $topic->info()['name']; // projects/my-awesome-project/topics/my-new-topic