rabbitmq - the queue appears to be empty even though there are messages on it

Question

I have a rabbit consumer that works fine (it correctly processes messages, sends acks, etc.).

In it, I am trying to check if a queue is empty using the queue_declare and passive flag (as suggested here: RabbitMQ - How to check if queue is empty?).

The relevant part of the code looks like this:

  <?php
      $a = $this->channel->queue_declare($queueName, true);
      var_dump($a);

My problem is that it always returns (regardless on how many messages are on queue - I tested with like 1000):

    array(3) {
      [0]=>
        string(4) "fastQueue"
      [1]=>
        int(0)
      [2]=>
        int(1)
    }

Any idea why?