Braintree release notification not working

I used a Sandbox account for Braintree for recurring subscribers. Verification of the final URL of the web host completed successfully. But I didn't receive any push notifications on my server even after the subscription was successful. I am using PHP framework to get POST details. Below is the code segment.

<?php

require_once(app_classes.'DB.Class.php');
require_once(app_classes.'util.Class.php');
require_once app_classes.'braintree/lib/Braintree.php';
Braintree_Configuration::environment(bt_environment);
Braintree_Configuration::merchantId(bt_merchant_id);
Braintree_Configuration::publicKey(bt_public_key);
Braintree_Configuration::privateKey(bt_private_key);

if(
     isset($_POST["bt_signature"]) &&
     isset($_POST["bt_payload"])
) {
    $webhookNotification = Braintree_WebhookNotification::parse(
        $_POST["bt_signature"], $_POST["bt_payload"]
);

$message =    "[Webhook Received " . $webhookNotification->timestamp->format('Y-m-d H:i:s') . "] "
        . "Kind: " . $webhookNotification->kind . " | "
                . "Subscription: " . $webhookNotification->subscription->id . "\n";

$subDetails = array( 'data' => $message );
DB::insert('subscription_notifications', $subDetails);
}
return 200;
?>

      

        
        
        
      

    

The server is SSL certified. Please let me know if there is anything I need to do to make it work.