Updating database without form data in CakePHP - php

Question

This may be a no brainer for some of you but I'm really scratching my head on this one. I started using CakePHP and got hooked on it but it is different than regular PHP (they use some shorthand for a lot of coding). At any rate, I'm trying to update my database when a button is pressed without having the user to enter in new data in a form. Currently I'm using AJAX and a button tag that calls the javacript function. It's supposed to be as if you are registering for a class that will be added to "My Courses"later (populated by the enrolment [sic] table in the database). This is the AJAX code:
<script>
function loadXMLDoc()
{
var xmlhttp;
if (window.XMLHttpRequest)
{// code for IE7+, Firefox, Chrome, Opera, Safari
xmlhttp=new XMLHttpRequest();
}
else
{// code for IE6, IE5
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
document.getElementById("alert").innerHTML=xmlhttp.responseText;
}
}
xmlhttp.open("POST","/Enrolments/register.php",true);
xmlhttp.send();
}
</script>
In hindsight it seems silly to need the if statement that checks for IE6 and below but that was just the W3 recommendation. This is what what follows for the button:
<button onclick="loadXMLDoc()">Register</button>
And of course there is some content on the page (a video to preview the course). This is what the register.php looks like:
<?php
$user = $this->Session->read('Auth.User');
$id = $user['user_id'];
$this->Status->read(null, $id);
$this->Status->set('course_id', 1);
$this->Status->save();
echo "You are now registered for this course";
?>
I didn't add a look up for the correct course_id to be added. I was just trying to test it by passing a "1" for that field (currently the enrolment [sic] table has only two columns- user_id and course_id) to see if it would actually update the database. Presently, it does not. Any advice would be great.
Edit: To be clear, I'm trying to make it where the enrolments table is updated with the following information upon clicking "Register": user_id = the current user_id, course_id = 1.

Answer 1

From #Guillermo comment this seems not to be cakephp's way to do it, however when using post:
xmlhttp.open("POST","/Enrolments/register.php",true);
you need to at least set this header also:
http.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
But if you can live with the character limits of a simple get, just use it:
xmlhttp.open("GET","/Enrolments/register.php",true);