AJAX request path

[eluser]ptvw2011[/eluser]
I am using some AJAX in my Code Igniter application but it only seems to work when the PHP file which is carrying out some processing is referred to absolutely.

I want to use a relative path as I don't want to change individual URL s when migrating from my local server.

Code:

new Ajax.Request("http://localhost/pth/to/my/php/file",
        {
          method: 'post',
          postBody: 'name='+ $F('name'),
          onComplete: showResponse
        });
}

At present I have the PHP file saved in a folder located in the Code Igniter root which is the same place as this JavaScript file.

Where is the best place to save the PHP file and how would I refer to it relatively in Code Igniter?

I've tried using '/filename.php' which I suppose should work as its in the same directory as this JavaScript file.

Thanks

me