Determine when a php script has run, then continue in actionscript

Question

I  want to run a php script before continuing to processs any more a.s. because the php script creates an xml file on which the a.s. depends. In addition, I need to send a variable to the php script that tells it what kind of xml script it will create.
(i suppose I could just add the rest of the a.s. into a later frame but that sounds like a method when all else fails)
Here's how it works in detail:
1. someone clicks on the "oil" button which means they want to view all the images in the "oil" gallery.
2. A php script creates an xml that includes only the oil images (that''s the variable it must receive from Flash "oil").
3. The php script finishes its job before a.s. script continues
Here's how I started but I'm not even sure what type of "event" is used to listen for a finished php script or where to put the variable parameter:
oil_btn.addEventListener(MouseEvent.CLICK,xmlGenesis);
function xmlGenesis
(evt:MouseEvent):void{
    var urlRequest:URLRequest= new URLRequest
    ('xmlGenesis.php');
    navigateToURL (urlRequest);
thank you for your assistance
chop

Answer 1

Well, after a few hours of looking around the Internet and in reference books and at the example you indicated, I finally managed to nail the pieces together to get something that works... just before I decided to start drinking again. whew! It works!
var myVariables:URLVariables = new URLVariables();
myVariables.gallery = "nature";
var myURLRequest:URLRequest = new URLRequest("xmlGenesis.php");
myURLRequest.data = myVariables;
var ulLoader:URLLoader= new URLLoader();
ulLoader.addEventListener(Event.COMPLETE,xmlComplete);
ulLoader.load(myURLRequest);
function xmlComplete (ev:Event):void{ // finished creating xml file
do everything else
thank you
That which doesn't kill us makes us stronger.
Friedrich Nietzsche