Updating Mysql database data using Php and Ajax - php

Question

I'm trying to set up a button which when clicked adds a fixed number to the database value. The database is called var_stat and consists of id and value. The table has one row so far where id = var and value = 35. If clicked, the button should add 5 to the value making it 40.
I'm not sure what to do here, as all answers I found used a completely different approach and strings instead of integers. So far I have done this:
if (isset($_POST['n'])){
$n = $_POST['n'] ;
$stmt = $con->prepare('UPDATE var_stat SET value = value + $n WHERE id = ? ');
$stmt->bind_param('s', $id);
$id = "var";
$stmt->execute();
$stmt->close();
}
<script src="js/jQuery-3.4.1.js"></script>
<script>
function add(n){
$.ajax({
type: "POST",
url: "update.php",
data: {
'n' : n
},
cache: false,
success: function(response)
{
alert("Record successfully updated");
}
});
}
</script>
<form>
<input type="button" value="+5" class="btn btn-circle btn-grey col-sm-10" onclick="add(5);">
</form>
If I change $n in the update.php to an integer and run the update.php by itself it works, however I can't seem to get this to run through my html page, so I guess there's something wrong with my javascript code?

Answer 1

Bind the n as well, move the id before the binding statement
if (isset($_POST['n'])){
$n = $_POST['n'] ;
$id = "var";
$stmt = $con->prepare('UPDATE var_stat SET value = value + ? WHERE id = ? ');
$stmt->bind_param('is',$n, $id);
$stmt->execute();
$stmt->close();
}