ActionScript 3.0 :: Setting Communication Between Flash And PHP File?

I set up something using almost the same exact code as something else I got to establish communication between the flash file and a php file. But for some reason it's not working now. But the other one works, and the code is pretty much the same...

AS3 Code:
submit_btn.addEventListener(MouseEvent.CLICK, submit);
function submit(event:Event):void {
var variables:URLVariables = new URLVariables();
var varSend:URLRequest = new URLRequest("[URL]");
varSend.method = URLRequestMethod.POST;
[Code] .....

Yet every time I try it outputs the error message in the text field and the trace returns a undefined variable.