Sending problem via URL (GET)

Good morning friends, I have these buttons here:

    <button class="btn-excluir" data-id="1">Primeiro</button>
    <button class="btn-excluir" data-id="2">Segundo</button>
    <button class="btn-excluir" data-id="3">Terceiro</button>
    <button class="btn-excluir" data-id="4">Quarto</button>
    <button class="btn-excluir" data-id="5">Quinto</button>

And this simple script:

<script type='text/javascript'>

             var btn = document.getElementsByClassName('btn-excluir');

             for (var i = 0; i < btn.length; i++) {

                  btn[i].addEventListener('click', function (e) {

                  var a = e.target.dataset.id;

                  ajax_envio('script.php?variavel=' + a);

                        }, false);

               }

               function ajax_envio(arquivo) {

                   var http = new XMLHttpRequest;

                   http.open('GET', arquivo, true);
                   http.send();


                }

                        </script>

The PHP code, to get the value of the variable:

<?php $valor_da_variavel = $_GET['variavel']; 

      echo "Valor da variável: $valor_da_variavel";    ?>

The script basically takes the data-id button and sends this value to the variable $ variable_value .

But here's the problem: URL does not want to change, although when I press F12 and I go to NETWORK and I click one on some button, it shows in the Name column what it should look like in URL . It basically looks like this:

Name: script.php? variable = 2 (if I click the second button)

Status: 200 OK

Type: xhr

Initiator: .php: 72

Size: 2.5Kb

Time: 10ms

And as I'm using $ variable_value as a parameter in a function, it's returning null.

I wonder what the problem might be. As for PHP. the error message I have is:

Undefined index: variable_value (What is the $ variable_value);

Thank you in advance !!