php的json_parse从mongo游标中删除_id属性

Question

我在一个简单的集合上有一个mongo光标。给

echo '<pre>' . print_r($result,1);
echo json_encode($result, JSON_FORCE_OBJECT, 512);

我明白了

[0] => stdClass Object
        (
            [_id] => MongoDB\BSON\ObjectID Object
                (
                    [oid] => 5b54af32e04d704eb82bbba1
                )

            [name] => axlotl
            [age] => 99
            [email] => axlotl@somewhere.com
        )



{"0":{"_id":{},"name":"axlotl","age":"99","email":"axlotl@somewhere.com"}}

为什么json_encode()删除了对象ID？