Webhook always failed authentication in Authorize.Net

In Authorize.Net created the subscription and recurring events handling using webhook. When I test using the web hook account authentication works but in actual webhook notification always failed authentication

if (isset($this->header['x-anet-signature'])) {
        $json = Json::encode($this->body);
        if ($json) {
            //To check the header and signature is true
         if (hash_equals(strtolower($this->header['x-anet-signature']),
                    'sha512=' . hash_hmac('sha512',$json, $secret))
            ) { 


             }else{
                yii::info($json,'webhookhNotifications');
                throw new \yii\web\ServerErrorHttpException('Authentication failed in Webhook');
              return false;

            } 
        }
    }

Webhook JSON

{
   "notificationId":"4bbba8fb-1d32-46b6-a513-a9ca2fed885c",
   "eventType":"net.authorize.customer.subscription.created",
   "eventDate":"2019-11-27T06:20:36.3621687Z",
   "webhookId":"a2929d59-147e-4400-a2bb-b3bd25a0311d",
   "payload":{
      "name":"Test subscription",
      "amount":290.00,
      "status":"active",
      "profile":{
         "customerProfileId":1921894828,
         "customerPaymentProfileId":1834842681,
         "customerShippingAddressId":1879009509
      },
      "entityName":"subscription",
      "id":"6168233"
   }
}

Secret key

F7B582AFFA9372866965456CFAC0D1B1219258F955FD5266D1A96BF9BE3C85F7D54C7CDFF9EF3EE7D3916EACB5EE920167F557BBB307288C17FBD169F0257AB4

x-anet-signature

sha512=FDE5518801C115C4886311877B4C37F6C26ABACE01ADB973EF372FB51C8F1E5321A83717161AD7DEFFD46F5013900E68B6220F3B25E9302A4208A9C673D32749

Your code should work. I simplified it a bit for testing purposes but using the values you provided above it does indeed validate successfully:

$signature = 'sha512=FDE5518801C115C4886311877B4C37F6C26ABACE01ADB973EF372FB51C8F1E5321A83717161AD7DEFFD46F5013900E68B6220F3B25E9302A4208A9C673D32749';
$json = '{"notificationId":"4bbba8fb-1d32-46b6-a513-a9ca2fed885c","eventType":"net.authorize.customer.subscription.created","eventDate":"2019-11-27T06:20:36.3621687Z","webhookId":"a2929d59-147e-4400-a2bb-b3bd25a0311d","payload":{"name":"Test subscription","amount":290.00,"status":"active","profile":{"customerProfileId":1921894828,"customerPaymentProfileId":1834842681,"customerShippingAddressId":1879009509},"entityName":"subscription","id":"6168233"}}';
$secret = 'F7B582AFFA9372866965456CFAC0D1B1219258F955FD5266D1A96BF9BE3C85F7D54C7CDFF9EF3EE7D3916EACB5EE920167F557BBB307288C17FBD169F0257AB4';

if (hash_equals(strtolower($signature), 'sha512=' . hash_hmac('sha512', $json, $secret))) {
    echo 'valid';
}else{
    echo 'invalid';
}

Demo

I think your error is you are encoding the JSON which is already JSON. So change this line:

$json = Json::encode($this->body);

new code:

if (isset($this->header['x-anet-signature'])) {
        $json = $this->body;
        if ($json) {
            //To check the header and signature is true
         if (hash_equals(strtolower($this->header['x-anet-signature']),
                    'sha512=' . hash_hmac('sha512',$json, $secret))
            ) { 


             }else{
                yii::info($json,'webhookhNotifications');
                throw new \yii\web\ServerErrorHttpException('Authentication failed in Webhook');
              return false;

            } 
        }
    }