PHP: json_decode使用curl读取时返回NULL

我需要解码JSON片段，以便我可以开始操作JSON，但是json_decode返回null，当我尝试使用curl读取时。

$username='xx';
$password='xx';
$headers = array(
'Content-Type:application/json',
    'accept: application/json',
    'Authorization: Basic '. base64_encode($username.":".$password),
    'x-myobapi-exotoken: xx'
);
$url = "http://example.com";
$ch = curl_init();
curl_setopt($ch, CURLOPT_HEADER, true);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_USERPWD, $username.":".$password);
curl_setopt($ch, CURLOPT_URL, $url);
$output = curl_exec($ch);
echo '<pre>';
echo $output; //works fine till here
echo "<br><br>Break <br>";
//all the commented part has been tried
//$my_array = json_decode(str_replace (''"','"', $output), true);
//var_dump($my_array);
// print_r(json_decode(substr($output,3))); //tried already
$obj = json_decode($output);
var_dump($obj);//gives NULL
print $obj->{'firstname'};
curl_close($ch);

在Google Chrome中输出如下:

Output:
HTTP/1.1 200 OK
Content-Length: 675
Content-Type: application/json; charset=utf-8
Server: Microsoft-HTTPAPI/2.0
Link: ; rel="describedBy"; method="PUT",; rel="describedBy"; method="POST"
Date: Tue, 12 Aug 2014 07:30:48 GMT
{
  "jobtitle": null,
  "firstname": "XYZ",
  "lastname": "XYZ",
  "fullname": "XYZ XYZ",
  "directphonenumber": null,
  "mobilephonenumber": null,
  "email": null,
  "postaladdress": {
    "line1": "",
    "line2": "",
    "line3": "",
    "line4": "",
    "line5": ""
  },
  "postalcode": "",
  "deliveryaddress": {
    "line1": "",
    "line2": "",
    "line3": "",
    "line4": "",
    "line5": "",
    "line6": ""
  },
  "advertsourceid": 0,
  "active": true,
  "optoutemarketing": false,
  "salespersonid": 10,
  "defaultcompanyid": null,
  "extrafields": [],
  "id": 129,
  "href": "http://example.com"
}
Break
NULL
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PHP代码中的注释将显示我已经尝试过的所有内容和所有可用的解决方案。