$sampleNotificationSignature = 'WITHHELD';
$sampleNotificationPayload = 'WITHHELD';
use Omnipay\Omnipay;
$gateway = Omnipay::create('Braintree');
$gateway->setPrivateKey('WITHHELD');
$gateway->setPublicKey('WITHHELD');
$gateway->setMerchantId('WITHHELD');
$gateway->setTestMode(TRUE);
$notification = $gateway->parseNotification([
    'bt_signature'  => $sampleNotificationSignature,
    'bt_payload'    => $sampleNotificationPayload
])->send();

Fatal error: Uncaught Braintree\Exception\Configuration: Braintree\Configuration::merchantId needs to be set (or accessToken needs to be passed to Braintree\Gateway). in /vendor/braintree/braintree_php/lib/Braintree/Configuration.php:254

Stack trace:
#0 /vendor/braintree/braintree_php/lib/Braintree/WebhookNotificationGateway.php(10): Braintree\Configuration->assertHasAccessTokenOrKeys()
#1 /vendor/braintree/braintree_php/lib/Braintree/Gateway.php(257): Braintree\WebhookNotificationGateway->__construct(Object(Braintree\Gateway))
#2 /vendor/braintree/braintree_php/lib/Braintree/WebhookNotification.php(35): Braintree\Gateway->webhookNotification()
#3 /vendor/omnipay/braintree/src/Gateway.php(281): Braintree\WebhookNotification::parse('DETAILS HIDDEN')
#4 braintree.php(27 in /vendor/braintree/braintree_php/lib/Braintree/Configuration.php on line 254

use Braintree_Gateway;
use Braintree_Configuration;
Braintree_Configuration::environment('sandbox');
Braintree_Configuration::merchantId('PRIVATE');
Braintree_Configuration::publicKey('PRIVATE');
Braintree_Configuration::privateKey('PRIVATE');

$sampleNotification = Braintree_WebhookTesting::sampleNotification(
    Braintree_WebhookNotification::SUBSCRIPTION_WENT_PAST_DUE,
    'my_id'
);

$webhookNotification = Braintree_WebhookNotification::parse(
    $sampleNotification['bt_signature'],
    $sampleNotification['bt_payload']
);

$webhookNotification->subscription->id;

print_r($webhookNotification);