Capturing ajax variables using php

Question

I have the following ajax :

$(".select").change(function()
{
  var select = $("#selectAparelho").val(); 
$.ajax({
  async: false,
  type: "POST", 
  url: "administrador.php",
  data: aparelho = select
}).done(function (data) {
  console.log(aparelho);
});
return false;
});

The problem is that when my php server checks to see if the variable is null:

if (isset($_POST['aparelho']))

The server checks that it is null because at that time it has not yet received the value, so it returns that it is null and then does not go through that chunk of code to do the check again. What I need is a way to get the server to go through that code location again to check if the variable is empty again, by that time it will already be with the value it received from ajax and then it will perform the operation I wish.