How do I enable cross-origin resource sharing on XAMPP?

I have a html file on my localhost with a form and jquery/ajax which handles the post data. A simple php script looks up the data in a mysql database table

This is the main part:

// $.post('lookup_update.php', $(this).serialize()) //<- local part which works   $.post('http://www.example.com/projectX/lookup_update.php', $(this).serialize()).done(function (data)                            { etc.

But when I point to the online lookup_update.php I get the following error message in chrome

XMLHttpRequest cannot load http://www.example.com/projectX/lookup_update.php. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost' is therefore not allowed access. The response had HTTP status code 404.

As I understand it I need to use

 header("Access-Control-Allow-Origin: *");

for php. But when I add this to example.com/lookup_update.php, the file gives a 404 when the localhost file tries to call it.

I also tried to add the following to my Xampp apache config file

Header set Access-Control-Allow-Origin "*"

How do I correctly enable cross-origin resource from my local XAMPP setup??

[EDIT]This is my simple form on my localhost

<!--Begin form--><div id="form" class="result"><form method="post" id="reg-form"  class="form-horizontal"><div class="controls"><input type="text" name="code" id="code" placeholder="Code"  class="form-control input-lg" />      </div></form></div><!--End form-->

With the following form jquery code

<script type="text/javascript">            $(document).ready(function ()            {                $(document).on('submit', '#reg-form', function ()                {                    var tmpCode = $("#code").val();//                    $.post('lookup_update.php', $(this).serialize())                      $.post('http://www.example.com/projectX/lookup_update.php', $(this).serialize())                            .done(function (data)                            {                                $("#reg-form").fadeOut('slow', function ()                                {                                    $(".result").fadeIn('slow', function ()                                    {                                        console.log("inner test "+ tmpCode);                                        $(".result").html(data);                                        setTimeout(function () {                                            location.reload();                                            $('input').val("");                                        }, 3000);                                    });                                });                            })                            .fail(function ()                            {                                alert('fail to submit the data');                            });                    return false;                });            });</script>

[EDIT 2]

OK, i don't think it has to do with the online lookup_update.php file, as I am using this to test in another file

            var testXHR = $.post("http://www.example.com/projectX/lookup_update.php", function (data) {            alert("success:"+ data);        })

And in the alert popup window I see the expected data