php空数组怎么转json对象

$arr  = [
     'id'  => 123.,
     'name'  =>  'andrew' ,
];
 
$jsonRet  = json_encode( $arr );
 
print_r( $jsonRet );

{
  "id": 123,
  "name": "andrew"
}

$arr = [];
$jsonRet = json_encode($arr);
print_r($jsonRet);

[
  
]

$arr = [];
$jsonRet = json_encode($arr, JSON_FORCE_OBJECT);
print_r($jsonRet);

$arr = [
    'jsonArray' => [
        '21', '12', '13'
    ],
    'jsonObject' => []
];
$jsonRet = json_encode($arr,JSON_FORCE_OBJECT);
print_r($jsonRet);

{
  "jsonArray": {
    "0": "21",
    "1": "12",
    "2": "13"
  },
  "jsonObject": {
    
  }
}

$bar = array();  
$foo = (object)$bar;  
echo json_encode($foo);

$arr = [
    'jsonArray' => [
        '21', '12', '13'
    ],
    'jsonObject' => new \ArrayObject()
];
$jsonRet = json_encode($arr);
print_r($jsonRet);

{
  "jsonArray": [
    "21",
    "12",
    "13"
  ],
  "jsonObject": {
    
  }
}