非法的字符串偏移量错误

我在运行脚本时遇到了几个错误：

警告：第35行/var/www/BrandonsBlog/comments.php中的非法字符串偏移量"photo_id"
注意：未定义的偏移量：2 in/var/www/BrandonsBlog/comments.php on line 35

我的代码：

    ini_set("display_errors", TRUE);
    include "Database.php";
    Class Database {
        protected $conn;
        public function setDb($conn){
            $this->conn = $conn;
        }
    }
    Class Images extends Database{
        protected $stmt;
        public function RetrieveImages(){
            $this->stmt = $this->conn->prepare('SELECT * FROM `pictures`');
            $this->stmt->execute();
            $boom = $this->stmt->fetchAll();
            return $boom;
        }
    }
    Class Content extends Images{
    }
    $test = new Images();
    $test->setDb($conn);
    $test2 = $test->RetrieveImages();
    var_dump($test2);
foreach ($test2 as $key => $value) {
    $photo_id = $value[$key]['photo_id'];
    //echo '<div class="hello" id="'.$photo_id.'"></div>';
}

var_dump($test2);给我以下输出：

array(3) {
  [0]=>;
  array(4) {
    ["id"]=>;
    string(1) "1"
    [0]=>;
    string(1) "1"
    ["photo_id"]=>;
    string(1) "1"
    [1]=>;
    string(1) "1"
  }
  [1]=>;
  array(4) {
    ["id"]=>;
    string(1) "2"
    [0]=>;
    string(1) "2"
    ["photo_id"]=>;
    string(1) "2"
    [1]=>;
    string(1) "2"
  }
  [2]=>;
  array(4) {
    ["id"]=>;
    string(1) "3"
    [0]=>;
    string(1) "3"
    ["photo_id"]=>;
    string(1) "3"
    [1]=>;
    string(1) "3"
  }
}

这就是我正在检索的数据库表的样子：

mysql> select * from pictures;
+----+----------+
| id | photo_id |
+----+----------+
|  1 | 1        |
|  2 | 2        |
|  3 | 3        |
+----+----------+
3 rows in set (0.00 sec)

有人能告诉我为什么我会出现这个错误吗？在我的var_dump()中，每行中的值似乎都是重复的，这可能是我的错误的原因？