javascript, ajax, PHP insert onSubmit not working...POST not set

javascript, ajax, PHP insert onSubmit not working...POST not set

I'm trying to insert into my MySQL database with an onSubmit() function...
For the main part, my function works. However, it keeps inserting a 1 into
my DB where the logbook_id field is (the logbook_id should NOT be a 1
every time). Here's my (relevant) html:
echo '<div class="logbook_box" id="logbook'.$id.'"
style="display: block; overflow: hidden;">';
echo '<form action="cgi-bin/read_log.php" method="post"
onSubmit="return ajaxSubmit(this, ' . $id . ');">' .
'<input type="hidden" value="'.$id.'" id="id" name="id" />'.
'<input style="float: right; width: 20%" type="submit" value="Mark as
Read" />' .
'</form>';
Here's the onSubmit function:
<script type="text/javascript" src="cgi-bin/jquery-2.0.3.js"></script>
<script>
var ajaxSubmit = function(form, id) {
div = document.getElementById("logbook" + id);
div.style.display = "none";
// fetch where we want to submit the form to
var url = $(form).attr('action');
var id = $('#id').val();
// setup the ajax request
$.ajax({
url: url,
data: { id : id },
dataType: "json",
success: function() {
if(rsp.success) {
alert('Marked as read, thanks!');
}
}
});
// return false so the form does not actually
// submit to the page
return false;
}
</script>
And here's my PHP file:
<?php
include('/var/www/olin2/includes/functions.php');
$con = connect_db();
session_start();
$id = isset($_POST['id']);
$username = $_SESSION['username'];
$query = 'insert into read_logbook(logbook_id, username)
values("'.$id.'", "'.$username.'")';
mysqli_query($con, $query) or die(mysqli_error($con));
?>
So, the <div> disappears like it should when Submit is clicked, which
shows that the id is getting passed correctly. The PHP script is called,
but it either doesn't input anything into the database, or (even more
strangely) it will submit my username and the value 1 into the database if
I change the PHP from $_POST to $_GET without changing the method in the
form.
I'm new at ajax/javascript, so I'm sure it has something to do with the
function, but I'm confused and at a complete loss... Any ideas?