How to have functional callback url

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PHP

website

WhatsApp

I need a functional callback on my website from whatsapp cloud API but am not getting it right and i need help. On whtasapp cloud, i added https://abc.com/whatsapp and verify token as 123 for example. On my whatsapp folder, i have index.php with the below php code, this is not showing anything. How do i verify whatsapp POST to my endpoint url with php? Do i need to add https://abc.com/whatsapp/webhook to callback url like this? All is not just working. Help

What I have tried:

<?php
if($_SERVER['REQUEST_METHOD']=="GET"){
    echo $_GET['hub_challenge']; //respond back hub_callenge key
    http_response_code(200);
}else{
    $data = json_decode(file_get_contents('php://input'), true);
    error_log(json_encode($data)); //print inbound message     
}