爲什麼json_encode/decode這樣配置我的數組？ PHP

我想發送一個JSON文件來做一些測試。我有一個簡單的測試文件來創建一個多維數組。下面是測試文件：爲什麼json_encode/decode這樣配置我的數組？ PHP

<?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; 
?>

我試圖檢索文件，像這樣：

<?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...

我得到關於類stdClass的對象的錯誤在我的if語句行。所以，我做了的var_dump（），這是結果：

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" 
     } 
    } 
}

我的問題是，爲什麼我不能訪問的信息，我做了json_decode（後），像這樣：