I took the following code from http://blog.vamapaull.com/?p=426 (with some minor adjustments), but keep getting
Error: Error #2101: The String passed to URLVariables.decode() must be a URL-encoded query string containing name/value pairs.
at Error$/throwError()
at flash.net::URLVariables/decode()
at flash.net::URLVariables()
at flash.net::URLLoader/onComplete()

Code:
stage.showDefaultContextMenu = false;

//path to the php file on your server
var phpPath:String = "login.php";

//make password text field (when you type the characters will be like "******")
pass.displayAsPassword = true;

lbutton.addEventListener(MouseEvent.MOUSE_DOWN, loginDown);

function loginDown(e:MouseEvent):void{
	//check to see if something in both the user and pass text fields
	if (user.text != "" && pass.text != "") {
		sendLoadData();
	}else{
		info.gotoAndStop("please");
	}
}
stop();


function sendLoadData():void
{
	var dataRequest:URLRequest = new URLRequest(phpPath);
	dataRequest.method = URLRequestMethod.POST;				
	
	// define the custom parameters that will be sent to the .php file
	var params:URLVariables = new URLVariables();
	params.user = user.text;
	params.pass = pass.text;
	
	dataRequest.data = params;
	
	var urlLoader:URLLoader = new URLLoader();
	urlLoader.dataFormat = URLLoaderDataFormat.VARIABLES;
	urlLoader.addEventListener(Event.COMPLETE, urlLoaderComplete);
	
	try
	{
		urlLoader.load(dataRequest);
	}
	catch (event:Error)
	{
		trace("Incorrect PHP file path.");
	}
}

function urlLoaderComplete(event:Event):void
{
	// once all the data has been sent and we'll check for a message sent
	// from the php file to tell us if the operation has ended with success or not
	errorHandler(event.target.data.secure_response);
}

function errorHandler(message:Number):void
{
	// check the message that was sent back from the php file
	//trace(message)
	if (message == 1)
	{
		MovieClip( parent ).editmc.gotoAndStop( "success" );
	}
	else
	{
		info.gotoAndStop("wrong");
	}
}
PHP:
Code:
<?
//Made by vamapaull: http://blog.vamapaull.com


//Your set username and password
$username = "user";
$password = "pass";


//Variables received from ActionScript
$user=$_POST['user'];
$pass=$_POST['pass'];


//Chack them against each other
if ($user == $username && $pass == $password){
	print "secure_response=1";
}else{
	print "secure_response=2";
}

?>