JSON: how to decode to array and use the value

Question

I am playing with a payment system around, which uses json. In result the user gets redirected to the checkout page. The PHP response in the end is something like this:

JSON:

{
  "id": 150, 
  "full_page_checkout": "https://swipe.lv/client/payment/979ffe8e8e74dfcce30c517aca07a5f848ddf8bf2e079080f8b623a2e59afdd8/full_page/", 
  "iframe_checkout": "https://swipe.lv/client/payment/979ffe8e8e74dfcce30c517aca07a5f848ddf8bf2e079080f8b623a2e59afdd8/iframe/", 
  "errors": [], 
  "timestamp": 1447668000
}

My code lookes like this:

PHP:

session_start();

$name = $_POST['name'];
$email= $_POST['email'];
$tel = $_POST['tel'];
$adress = $_POST['adress'];
$city = $_POST['city'];
$plz = $_POST['plz'];
$how = $_POST['howtext'];
$quantity = $_POST['quantity'];
$dispatch = $_POST['dispatch'];
$payment = $_POST['payment'];
$price = $_POST['pricevalue'];
$comment = $_POST['comment'];


$public_key = '4838026a073b766840dbae4c2c5cbfab';
$private_key = '0000000000000000000000000000000000000000000000000000000';
$timestamp = (string)time();

$params = json_encode(array(
        'client' => array('email' => 'email@email.com'),
        'products' => array(
            array(
                'description' => 'Standard Cube',
                'price' => 20.45,
                'quantity' => 1
            ),
        )
    )
);

$authorization_message = $timestamp.' POST /api/v0.5/payments/ '.$params;
$authorization = hash_hmac('sha256', $authorization_message, $private_key);
$authorization_header = $public_key.','.$timestamp.','.$authorization;

$options = array(
    'http' => array(
        'header'  => "Content-type: application/json\r\nAuthorization: ".$authorization_header,
        'method'  => 'POST',
        'content' => $params,
        'ignore_errors' => true
    ),
);
$context = stream_context_create($options);
$result = file_get_contents('https://swipe.lv/api/v0.5/payments/ ', false, $context);
$link = substr($result,37,110);
echo json_encode($link);

To execute the link, I used a solution like this:

$link = substr($result,37,110);
echo json_encode($link);

Since the json changes depending on the values, I can't use the substr function. What I want to do is convert the jason into an array and than echo the value of "full_page_checkout"

How can I achieve this?

EDIT:

By using json_decode and echoing the result, I get just Object : object as result