change modal content with results from php file

I'm trying to dynamically change a modal's content with the result of a query using PHP.

So far, I've managed to reach the ajax call, but I'm not getting a success nor error message

PHP: get_user.php

<?php 
include ("session-connection.php");
session_start();

$result = $_POST['param'];
echo $result;
?>

HTML

<script type="text/javascript">
    function view_progress(row){
        console.log("id: " + row);
        var param = row;
        $.ajax({
            data: param,
            url: 'get_user.php',
            type: 'post',
            beforeSend: function(){
                $("#progressModal").modal('show');
                $("#alert_name").html("...");
            },
            error: function (){
                $("#alert_name").html("Error");
            },
            success: function(resultado){
                $("#alert_name").html(resultado);
            }
        }); 
    }
</script>

If the code is successfull it should change the label alert_name with the id the javascript function recieves.