How to get Webhook response using PHP

I'm creating a script that will receive response from Zoho Sign Webhook. I use the following condition to trigger the Webhook :

I am able to receive the hit into the Callback URL by saving every response into the database. I'm using a simple code below :

<?php 

@$server=$_SERVER;
@$post=$_POST;
@$get=$_GET;
$data = file_get_contents('php://input');
if(empty($post)){
    $post="emptypost-";
}

if(empty($get)){
    $get="emptyget";
}

$server=json_encode($_SERVER);

$q=mysqli_query($conn,"INSERT INTO `response` VALUES ('','".$data.$post.$get.$server."')");
if($q){
    echo "success";
}else{
    echo "failed".mysqli_error($conn);
}

?>

But i always get empty response using the script above. The $data, $post, or $get variable is always empty.

I'm trying to find the documentation on the Zoho but i only can get this discussion here. Which is not solving my problem.

Any other way to get the response from Zoho Sign Webhook using PHP?