Python/PHP/JS对象与json数据的转换key顺序问题

<?php
// obj -> json
$obj1 = [
    'name'=>'Tom',    
    'age'=> 23,
];
$obj2 = [
    'age'=> 23,
    'name'=>'Tom',
];
print_r(json_encode($obj1));
// {"name":"Tom","age":23}
print_r(json_encode($obj2));
// {"age":23,"name":"Tom"}
// json -> obj
print_r(json_decode('{"name":"Tom","age":23}'));
// ([name] => Tom [age] => 23)
print_r(json_decode('{"age":23,"name":"Tom"}'));
// ([age] => 23 [name] => Tom)

import json
# obj -> json
obj1 = {"age":23,"name":"Tom"}
print(json.dumps(obj1))
# {"age": 23, "name": "Tom"}
obj2 = {"name":"Tom", "age":23}
print(json.dumps(obj2))
# python3.7 {"name": "Tom", "age": 23}
# python3.6.5 {"name": "Tom", "age": 23}
# python2.7 {"age": 23, "name": "Tom"}
# json -> obj
arr1 = json.loads('{"age":23,"name":"Tom"}')
print(arr1)
# {'age': 23, 'name': 'Tom'}
arr2 = json.loads('{"name":"Tom", "age":23}')
print(arr2)
# python3.7 {'name': 'Tom', 'age': 23}
# python3.6.5 {"name": "Tom", "age": 23}
# python2.7 {u'age': 23, u'name': u'Tom'}