将文件从闪存上传到服务器PHP

我要将文件从闪存上传到服务器。当用户开始时，输入他的用户名，然后我将其发送到php，这样：

    var myusername:String = username.text;
    username.restrict = "A-Za-z0-9";
    login_btn.addEventListener(MouseEvent.CLICK,login);
    function login (evt:MouseEvent):void{
    var loader : URLLoader = new URLLoader();
    var request:URLRequest = new URLRequest("http://domain/uploads/upload.php");
    request.method = URLRequestMethod.POST;
    var variables:URLVariables = new URLVariables();
    var userid:String = myusername;
    variables.ID = userid;
    request.data = variables;
    loader.load(request);
}

上传.php ：

<?php
$myuser = $_POST['ID'];
$uploads_dir = './uploads/'.$myuser;
if( $_FILES['Filedata']['error'] == 0 ){
    if( move_uploaded_file( $_FILES['Filedata']['tmp_name'],
         $uploads_dir.$_FILES['Filedata']['name'] ) ){
        exit();
    }
}
echo 'error';
exit();
?>

问题是文件上传到上传文件夹不在用户文件夹中。有人可以帮我吗？