AJAX posting to CakePHP controller action

I am posting some form values used for registration into a cakePHP controller action. The code is like this

var params='firstname='+encodeURIComponent(firstname) +'&lastname='+encodeURIComponent(lastname)+'&emailid='+encodeURIComponent(emailid)+'&password='+encodeURIComponent(password);
 
 xmlhttp.open('POST', url, true);
 xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
 xmlhttp.send(params);
 

this is doing the posting and I can see it in the Firebug in the POST section source as

firstname=name&lastname=name&emailid=mymailid&password=123123
 

But when I print $this->data in the action , it is not not showing any values. I have even used $_POST and it is also returning anything..

What I done wrong here..?

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