unserialize（）后奇数组键

Question

So I had some output in this format:

array (
  'job' => 'Illuminate\\Queue\\CallQueuedHandler@call',
  'data' => 
  array (
    'commandName' => 'App\\Jobs\\SampleJob',
    'command' => 'O:21:"App\\Jobs\\SampleJob":5:{s:10:"' . "\0" . '*' . "\0" . 'post";O:45:"Illuminate\\Contracts\\Database\\ModelIdentifier":2:{s:5:"class";s:18:"App\\Models\\Post";s:2:"id";i:28711;}s:10:"connection";N;s:5:"queue";N;s:5:"delay";N;s:6:"' . "\0" . '*' . "\0" . 'job";N;}',
  ),
)  

I need to access the id value (28711) under command.

So I unserialized it:

$output = (array) unserialize($event->data['data']['command']);

Now, when I print the array keys of this array, it gives me:

array (
  0 => '' . "\0" . '*' . "\0" . 'post',
  1 => 'connection',
  2 => 'queue',
  3 => 'delay',
  4 => '' . "\0" . '*' . "\0" . 'job',
)  

How am I supposed to access the first key's values?