Looking in https://github.com/videlalvaro/php-amqplib/blob/master/PhpAmqpLib/Channel/AMQPChannel.php

queue_bind seems to have a bunch of parameters:
```
 public function queue_bind($queue, $exchange, $routing_key="",
        $nowait=false, $arguments=null, $ticket=null)
```
I'm still trying to figure out the role of the routing_key. It appears that passing an empty value is OK but an actual value is significant as it's used in the queueBind call in the method. 

Thoughts?