Usa LoadVars:
Código PHP:
var theXml:XML = new XML();
var send_lv:LoadVars = new LoadVars();
send_lv.campo1 = "Hola mundo";
theXml.ignoreWhite = true;
theXml.onLoad = function() {
trace(theXml.firstChild.childNodes[0].firstChild.nodeValue);
};
send_lv.sendAndLoad("datos.php?nocache="+Math.random()*300,theXml,"POST");