Ajax POST with jQuery throwing empty errors

i'm trying to fix the following, simple code:

$(document).ready(function()
{
    $("#login").click(function()
    {
        var a = 3;
        var b = 2;
        var dataString = "a="+a+"&b="+b;
        console.log(dataString);
        $.ajax({
                    url: "ajax.php",
                    type: "post",
                    data: dataString,
                    cache:"false",
                    beforeSend: function(){console.log("Sending...");},
                    error: function(x,y,z){console.log("err: "+x+" "+y+" "+z);},
            success: function(x,y,z){console.log("succ: "+x+" "+y+" "+z);}
                });
    });
}); 

ajax.php:

<?php
$a = $_POST['a'];
$b = $_POST['b'];

echo $a+$b;
?>

My problem is that, the site always returns with error and emtpy x,y,z.