Extract data from HTTP_Request2_Response object using PHP

I am currently working on the response from the Microsoft Emotion API. In the API, HTTP/Request2.php is used and I successfully get the HTTP response message by using getBody() in HTTP_Request2_Response(Manual here). Just like the API manual, I do the following:

try
{
    $response = $request->send();
    echo $response->getBody();
}

What I got is like this:

[
  {
    "faceRectangle": {
      "height":264,
      "left":162,
      "top":523,
      "width":264,

    },
    "scores": {
      "anger":1.38974974E-06,
      "contempt":2.75673688E-08,
      "disgust":3.75520244E-06,
      "fear":5.69216375E-11,
      "happiness":0.999994636,
      "neutral":1.77765841E-07,
      "sadness":3.03275627E-09,
"surprise":2.25669652E-08
    }
  }
]

However, I only want some aspect of the "scores", like "anger" or "happiness". I tried to use json_decode on $response but I got an error:json_decode() expects parameter 1 to be string, object given. It seems that HTTP_Request2_Response provide me an object. How do I extract the data I want from the response?

Here is the var_dump of $response for your reference:

object(HTTP_Request2_Response)#5 (9) { 
["version":protected]=> string(3) "1.1" 
["code":protected]=> int(200) 
["reasonPhrase":protected]=> string(2) "OK" 
["effectiveUrl":protected]=> string(65) "https://westus.api.cognitive.microsoft.com/emotion/v1.0/recognize" 
["headers":protected]=> array(10) { 
["cache-control"]=> string(8) "no-cache" 
["pragma"]=> string(8) "no-cache" 
["content-length"]=> string(3) "274" 
["content-type"]=> string(31) "application/json; charset=utf-8" 
["expires"]=> string(2) "-1" 
["x-powered-by"]=> string(7) "ASP.NET" 
["apim-request-id"]=> string(36) "96fgh4z1-62z0-43r3-82z7-8823hdg5g86d" 
["strict-transport-security"]=> string(44) "max-age=31536000; includeSubDomains; preload" 
["x-content-type-options"]=> string(7) "nosniff" 
["date"]=> string(29) "Tue, 06 Mar 2018 14:55:20 GMT" } 
["cookies":protected]=> array(0) { } 
["lastHeader":protected]=> string(4) "date" 
["body":protected]=> string(274) "
[{"faceRectangle":{"height":264,"left":162,"top":523,"width":264},
"scores":{"anger":1.38974974E-06,"contempt":2.75673688E-08,"disgust":3.75520244E-06,"fear":5.69216375E-11,"happiness":0.999994636,"neutral":1.77765841E-07,"sadness":3.03275627E-09,"surprise":2.25669652E-08}}]" 
["bodyEncoded":protected]=> bool(true) 

$jsonString = $response->getBody();
$bodyAsArray = json_decode($jsonString, true); // true returns associative array
echo $bodyAsArray['scores']['anger'];