无法打印简单的JSON？

I know it's basic, but I couldn't get what's wrong because it's not my area. I am sending some request to a server and print the response like this:

$rest = curl_init();  
 curl_setopt($rest,CURLOPT_URL,$url);  
 curl_setopt($rest,CURLOPT_GET,1);  
 curl_setopt($rest,CURLOPT_HTTPHEADER,$headers);  
 curl_setopt($rest,CURLOPT_SSL_VERIFYPEER, false);  
 curl_setopt($rest,CURLOPT_RETURNTRANSFER, true);  
 $response = curl_exec($rest);  
 $json = json_decode($response, true);

 echo $response;

Where I get this:

{"results":[{"Devices":["52401E7E-C5D7","AE80C0F8-999E","764BFD92-9753","78A23379-2C14","EEA03545-5EB9","E18DDFEC-C3C9"],"UserID":"433011AC-228A-4931-8700-4D050FA18FC1","createdAt":"2015-11-04T15:06:33.564Z","objectId":"3os7BGxRoG","updatedAt":"2015-11-04T17:08:57.635Z"}]}

Then, I am trying to print the JSON or its fields, but I then get nothing:

  echo $json;
  echo $json['UserID'];
  echo $json['Devices'];

To clarify the comments a bit:

$str = '{"results":[{"Devices":["52401E7E-C5D7","AE80C0F8-999E","764BFD92-9753","78A23379-2C14","EEA03545-5EB9","E18DDFEC-C3C9"],"UserID":"433011AC-228A-4931-8700-4D050FA18FC1","createdAt":"2015-11-04T15:06:33.564Z","objectId":"3os7BGxRoG","updatedAt":"2015-11-04T17:08:57.635Z"}]}';
$json = json_decode($str, true); // to an associative array
// to echo the UserID
echo "userID : " . $json['results'][0]['UserID'];
// output: userID : 433011AC-228A-4931-8700-4D050FA18FC1

// to get the structure of the json array in general use print_r() as AbraCadaver pointed out
print_r($json);

In your attempt you were missing the results[0] part.