AS3 - Cancel PHP request

That script below is for uploading an image via PHP. Now I'ld like to give the user the option to cancel the upload. How to cancel the PHP request once it got send?

    package
{
    import com.adobe.images.PNGEncoder;
    import flash.display.BitmapData;
    import flash.display.MovieClip;
    import flash.events.Event;
    import flash.events.SecurityErrorEvent;
    import flash.events.IOErrorEvent
    import flash.net.URLLoader;
    import flash.net.URLLoaderDataFormat
    import flash.net.URLRequest;
    import flash.net.URLRequestMethod;
    import flash.utils.ByteArray;

    public class ServeruploadExample
    {
        private var loader:URLLoader;

        private const API_KEY:String = "<api key>";
        private const UPLOAD_URL:String = "http://example.com/upload-image.php";

        public function ImgurExample() {

            loader = new URLLoader();
            loader.dataFormat = URLLoaderDataFormat.VARIABLES;
            loader.addEventListener(Event.COMPLETE, onCookieSent);
            loader.addEventListener(SecurityErrorEvent.SECURITY_ERROR, onSecurityError);
            loader.addEventListener(IOE开发者_StackOverflow社区rrorEvent.IO_ERROR, onIOError);

            // Create a bitmapdata instance of a movieclip on the stage.
            var mc:MovieClip;
            var b:BitmapData = new BitmapData(mc.width, mc.height, true);
            b.draw(mc);
            var png:ByteArray = PNGEncoder.encode(b);

            var vars:String = "?key=" + API_KEY + "&name=name&title=title";
            var request:URLRequest = new URLRequest(UPLOAD_URL + vars);
            request.contentType = "application/octet-stream";
            request.method = URLRequestMethod.POST;
            request.data = png;

            loader.load(request);
        }
        // privates


    }
}

Just call the .close() method of the URLloader class, which will terminate the load operation.

In your example, set up a cancel button and:

public function cancel_upload_click_handler(e:MouseEvent):void
{
   loader.close();
}