No "reply_to" property. PHP rabbitmq

Following this RabbitMQ tutorial on how to create a rpc server and client in php me I found a problem.

It gives me the following problem, however I can not fully understand it.

The code I use is the following:

Server:

<?php
require_once __DIR__ . '/vendor/autoload.php';
use PhpAmqpLib\Connection\AMQPStreamConnection;
use PhpAmqpLib\Message\AMQPMessage;
$connection = new AMQPStreamConnection('localhost', 5672, 'admin', 'admin');
$channel = $connection->channel();
$channel->queue_declare('rpc_queue', false, false, false, false);
function fib($n) {
    if ($n == 0)
        return 0;
    if ($n == 1)
        return 1;
    return fib($n-1) + fib($n-2);
}
echo " [x] Awaiting RPC requests\n";
$callback = function($req) {
    $n = intval($req->body);
    echo " [.] fib(", $n, ")\n";
    $msg = new AMQPMessage(
        (string) fib($n),
        array('correlation_id' => $req->get('correlation_id'))
        );
    $req->delivery_info['channel']->basic_publish(
        $msg, '', $req->get('reply_to'));
    $req->delivery_info['channel']->basic_ack(
        $req->delivery_info['delivery_tag']);
};
$channel->basic_qos(null, 1, null);
$channel->basic_consume('rpc_queue', '', false, false, false, false, $callback);
while(count($channel->callbacks)) {
    $channel->wait();
}
$channel->close();
$connection->close();
?>

Client:

<?php
require_once __DIR__ . '/vendor/autoload.php';
use PhpAmqpLib\Connection\AMQPStreamConnection;
use PhpAmqpLib\Message\AMQPMessage;
class FibonacciRpcClient {
    private $connection;
    private $channel;
    private $callback_queue;
    private $response;
    private $corr_id;
    public function __construct() {
        $this->connection = new AMQPStreamConnection(
            'localhost', 5672, 'admin', 'admin');
        $this->channel = $this->connection->channel();
        list($this->callback_queue, ,) = $this->channel->queue_declare(
            "", false, false, true, false);
        $this->channel->basic_consume(
            $this->callback_queue, '', false, false, false, false,
            array($this, 'on_response'));
    }
    public function on_response($rep) {
        if($rep->get('correlation_id') == $this->corr_id) {
            $this->response = $rep->body;
        }
    }
    public function call($n) {
        $this->response = null;
        $this->corr_id = uniqid();
        $msg = new AMQPMessage(
            (string) $n,
            array('correlation_id' => $this->corr_id,
                  'reply_to' => $this->callback_queue)
            );
        $this->channel->basic_publish($msg, '', 'rpc_queue');
        while(!$this->response) {
            $this->channel->wait();
        }
        return intval($this->response);
    }
};
$fibonacci_rpc = new FibonacciRpcClient();
$response = $fibonacci_rpc->call(30);
echo " [.] Got ", $response, "\n";
?>