Can't figure out how to turn PHP variable into JQuery output

[nxzmplty]

Right, I apologise well in advance for coming to you all with the most simple question you've probably ever seen. I can not for the life of me get my head around this.

PHP Code:

<?php 

// this is ajax.php

$newRateNum = "1";    
echo json_encode($newRateNum);

?>

PHP Code:

// this is index.php

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js" type="text/javascript" charset="utf-8"></script>

        <script type="text/javascript" charset="utf-8">

            // What we want to do here is
            // Grab the ID
            // Generate the variables
            // Get a return from the PHP
            // Update $('span#'+id+' a.rateNum') with that return

    $(document).ready(function(){
                
        $("a.rateDn").click(function() {  
        // we want to store the values from the form input box, then send via ajax below  
        $.ajax({  
            type: "POST",  
            url: "ajax.php",  
            data: "id="+this.parentNode.id+"&v=d",  
            success: function(){  
            $('a.rateNum').html($newRateNum);
            }  
            });  
        return false;  
        });  
    });      



        </script>

        <span class="rate" id="i79341"><a href="#" class="rateDn">[-]</a><a class="rateNum">0</a><a href="#" class="rateUp">[+]</a></span> 


Seriously, clearly the problem is with the "$('a.rateNum').html($newRateNum);". All I want to know is, what the hell goes in the second set of brackets?

I simply want to replace the existing "0" in a.rateNum with whatever I output as $newRateNum in PHP.

Again, sorry because I know this is a question fit for a 6 year old - I just kind of suck at the whole client side coding

EDIT: Sticking an alert("whatever") in the success{} bit will do its job correctly, so I am getting that far for sure.

[devnull69]

The sticking point is that you cannot just presume that the variable name from PHP will be available to jQuery/Javascript "just like that".

PHP creates output, this output is sent to the browser and will then automatically be available as the first parameter of the success callback

        $.ajax({  
            type: "POST",  
            url: "ajax.php",  
            data: "id="+this.parentNode.id+"&v=d",  
            success: function(data){  
               var newRateNum = data;
               $('a.rateNum').html(newRateNum);
            }  
            });

[nxzmplty]

Thank you so very much. I'm sure I'll have some more questions in good time but this is exactly what I needed to progress