为什么json_encode/decode以这种方式配置我的数组?PHP

我正在尝试发送一个JSON文件来进行一些测试。我有一个简单的测试文件，它创建了一个多维数组。这是测试文件：

<?php
error_reporting(E_ALL);
ini_set('display_errors', '1');
$user['Mongo'] = null; 
$user['Facebook'] = 12345;
$user['Twitter'] = null;
$user['Foursquare'] = null;
$user['Google'] = null;
$user['Name'] = "Bill Gates";
$user['Sex'] = 'M';
$user['Age'] = 26;
$user['Birthday'] = "1985-08-13";
$user['Friends'][0]['Mongo'] = null;
$user['Friends'][0]['Facebook'] = 123456;
$user['Friends'][0]['Twitter'] = null;
$user['Friends'][0]['Foursquare'] = null;
$user['Friends'][0]['Google'] = null;
$user['Friends'][0]['Name'] = "John Smith";
$user['Friends'][0]['Relationship'] = "Open";
$user['Friends'][1]['Mongo'] = null;
$user['Friends'][1]['Facebook'] = 1234567;
$user['Friends'][1]['Twitter'] = null;
$user['Friends'][1]['Foursquare'] = null;
$user['Friends'][1]['Google'] = null;
$user['Friends'][1]['Name'] = "Martina McBride";
$user['Friends'][1]['Relationship'] = "Open";
$user_json = json_encode($user);
$call = curl_init('http://MY_IP_HERE/user_login.php');
curl_setopt($call, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($call, CURLOPT_POSTFIELDS, $user_json);
curl_setopt($call, CURLOPT_RETURNTRANSFER, true);
curl_setopt($call, CURLOPT_HTTPHEADER, array('Content-Type: application/json', 'Content-Length: '.strlen($user_json)));
$result = curl_exec($call);
curl_close($call);
echo $result;
?>

我正在尝试检索这样的文件：

<?php
include_once('interaction_class.php');
error_reporting(E_ALL);
ini_set('display_errors', '1');
$fp = fopen('php://input', 'r');
$rawData = stream_get_contents($fp);
$user = json_decode($rawData);
if ($user['Mongo'] == null)
{
    $user_id = $interaction->new_guest($user);
}
//...Other stuff...

我在if语句行中收到一个关于类stdCLass的Object的错误。所以，我做了一个var_dump（），结果是：

object(stdClass)#1 (10) 
{ 
    ["Mongo"] => NULL 
    ["Facebook"] => int(12345) 
    ["Twitter"] => NULL 
    ["Foursquare"] => NULL 
    ["Google"] => NULL 
    ["Name"] => string(15) "Bill Gates" 
    ["Sex"] => string(1) "M" 
    ["Age"] => int(26) 
    ["Birthday"] => string(10) "1985-08-13" 
    ["Friends"] => array(2) 
    { 
        [0] => object(stdClass)#2 (7) 
        { 
            ["Mongo"] => NULL 
            ["Facebook"] => int(123456)
            ["Twitter"] => NULL 
            ["Foursquare"] => NULL 
            ["Google"] => NULL 
            ["Name"] => string(10) "John Smith" 
            ["Relationship"] => string(4) "Open" 
        } 
        [1] => object(stdClass)#3 (7) 
        { 
            ["Mongo"] => NULL 
            ["Facebook"] => int(1234567) 
            ["Twitter"] => NULL 
            ["Foursquare"] => NULL 
            ["Google"] => NULL 
            ["Name"] => string(15) "Martina McBride" 
            ["Relationship"] => string(4) "Open" 
        } 
    } 
}

我的问题是，为什么我在做了json_decode（）之后不能访问信息，就像这样：

$thing['Key']

为什么它把它解码成一个Object而不是一个数组？

提前感谢您的帮助！