phpredis-Redis::subscribe(）期望参数2是一个有效的回调

我在使用subscribe方法时遇到了问题。任何帮助都是受欢迎的，以使它发挥作用。下面的php单元测试给出了以下错误。

这是phpredis提供的例子

https://github.com/nicolasff/phpredis#subscribe

Redis::subscribe() expects parameter 2 to be a valid callback, function 'f' not found or invalid function name
/myproj/test/RedisEventBusTest.php:37

RedisEventBusTest.php

<?php
namespace bkcqrs'eventing;
use bkcqrs'DefaultDomainEvent;
use bkcqrs'serializing'JsonSerializer;
class RedisEventBusTest extends 'PHPUnit_Framework_TestCase
{
    private $redisEventBus;
    private $redisSubscribeClient;
    public function setUp()
    {
        $serializer = new JsonSerializer();
        $this->redisEventBus = new RedisEventBus($serializer);
        $this->redisSubscribeClient = new 'Redis();
    }
    public function it_publishes_the_event_on_redis()
    {
        $notificationCountChangedEvent = new NotificationCountChangedEvent(array('userId' => 6, 'count' => 21));
        function f($redis, $channel, $msg)
        {
            var_dump($msg);
            $notification = json_decode($msg);
            var_dump($notification);
            $this->assertEquals($notification["userId"], $notificationCountChangedEvent->userId);
            $this->assertEquals($notification["count"], $notificationCountChangedEvent->count);
        }
        $this->redisSubscribeClient->subscribe(array("NotificationCountChanged"), 'f');
        $this->redisEventBus->publish($notificationCountChangedEvent);
    }
}
class NotificationCountChangedEvent extends DefaultDomainEvent
{
    public $userId;
    public $count;
}