<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
      <script type="text/javascript">
         function onClick(arg){
             alert("Dziękujemy za Twój głos!");
              $.ajax({
                  type: 'POST',
                  contentType: text,
                  url: 'data.php',
                  data: {
                     'arg': arg,
                  },
                  success: function(response) {
                     content.log(response);
                  }
              });

         }
      </script>
<?php
$servername = "xx";
$username = "xx";
$password = "xx";
$name ="xx";

$conn = new mysqli($servername, $username, $password, $name);

$id = $_POST;

$sql = "UPDATE Buttons SET SUMA = SUMA + 1 WHERE ID = '$id'";
$conn->query($sql);

$conn->close();
?>