未知錯誤執行命令（空文件返回），使用PHP

MongoDB中執行聚集時，我想組在MongoDB中：我下面的代碼：未知錯誤執行命令（空文件返回），使用PHP

$m = new MongoClient("localhost"); 
$c = $m->selectDB("grl_db")->selectCollection("USER_TBL"); 
$data = array (
    'title' => 'this is my title', 
    'author' => 'bob', 
    'posted' => new MongoDate, 
    'pageViews' => 5, 
    'tags' => array ('fun', 'good', 'fun'), 
    'comments' => array (
     array (
     'author' => 'joe', 
     'text' => 'this is cool', 
    ), 
     array (
     'author' => 'sam', 
     'text' => 'this is bad', 
    ), 
    ), 
    'other' =>array (
     'foo' => 5, 
    ), 
); 
$d = $c->insert($data, array("w" => 1)); 

$ops = array(
    array(
     '$project' => array(
      "author" => 1, 
      "tags" => 1, 
     ) 
    ), 
    array('$unwind' => '$tags'), 
    array(
     '$group' => array(
      "_id" => array("tags" => '$tags'), 
      "authors" => array('$addToSet' => '$author'), 
     ), 
    ), 
); 
$results = $c->aggregate($ops);

但犯錯時：