Use {@see \Google\Cloud\PubSub\Subscription::acknowledge()} to acknowledge
a single message.
Example:
$messages = $subscription->pull();
$messagesArray = iterator_to_array($messages);
$subscription->acknowledgeBatch($messagesArray);
See also:
https://cloud.google.com/pubsub/docs/reference/rest/v1/projects.subscriptions/acknowledge Acknowledge Message