passing params through ajax to php - php

Question

I know this is a very basic question, but i couldnt figure how to fix the code even after crawling the web for past 1 hour.
I have an unordered list containing the information about the categories in the database, with cat_id as primary key. and a subject table with cat_id as its foreign key, so i want to access the subjects table through ajax request for given category ID. below is the code i used to generate categories. Where i am stuck is, i dont know which DOM element to fetch in order to send the unique id in the url parameter ..
thanks ..
<ul id="search_form">
<?php
$cat = Category::find_all();
foreach($cat as $category) {
echo '<li id="';
echo $category->cat_id;
echo '"><a href="subject.php?id=';
echo $category->cat_id;
echo'">';
echo $category->category;
echo '</a></li>';
}
?>
</ul>
<div id="results">
<!-- ajax contents goes here -->
</div>
the ajax file is
window.onload = init;
function init() {
if (ajax) {
if (document.getElementById('results')) {
document.getElementById('search_form').onclick = function() {
ajax.open('get', 'subject.php?id='+id ); // subject.php?id=
// how will i pass the variable
ajax.onreadystatechange = function() {
handleResponse(ajax);
}
ajax.send(null);
return false;
}
}
}
}
function handleResponse(ajax) {
if (ajax.readyState == 4) {
if ((ajax.status == 200) || (ajax.status == 304) ) {
var results = document.getElementById('results');
results.innerHTML = ajax.responseText;
results.style.display = 'block';
}
}
}
and the subject.php
<?php
//include("tpl/header.php");
include("includes/initialize.php");
?>
<h2></h2>
<?php
if (isset($_GET['id'])) {
$id= mysql_real_escape_string($_GET['id']);
$subject = Subject::find_subject_for_category($id);
foreach($subject as $subj) {
echo $subj->subject_title;
}
} else {
echo "No ID Provided";
}
?>

Answer 1

I have used Jquery to do these kind of things and it works fine for me.
$.ajax({
type: GET,
url: "subject.php",
data: {id: $('#search_form :selected').val()},
success: function(result){
// callback function
}
});