callback.php

<?php                                    

require "config.php";                                   
$method = $_SERVER['REQUEST_METHOD'];                             
   
// In PHP, dots and spaces in query parameter names are converted to 
// underscores automatically. So we need to check "hub_mode" instead
//  of "hub.mode".                                                      
if ($method == 'GET' && $_GET['hub_mode'] == 'subscribe' &&       
    $_GET['hub_verify_token'] == VERIFY_TOKEN) {
  echo $_GET['hub_challenge'];
  exit;
} else if ($method == 'POST') {                         	         
  $updates = json_decode(file_get_contents("php://input"), true); 
  // Replace with your own code here to handle the update 
  // Note the request must complete within 15 seconds.
  // Otherwise Facebook server will consider it a timeout and 
  // resend the push notification again.
  mail('venu@riktamtech.com', 'FB real updates', $updates); 
  error_log('updates = ' . print_r($updates, true), 3, "updates.log");
}