The getUIComponentBitmapData method can create bitmapdata for a given IUIComponent. Pass any UIcomponent to get its respective bitmapdata.

public static function getUIComponentBitmapData(target:IUIComponent):BitmapData {      
    var resultBitmapData:BitmapData = new BitmapData(target.width, target.height);     
    var m:Matrix = new Matrix();     
    resultBitmapData.draw(target, m);     
    return resultBitmapData; 
}

Now convert the bitmapdata to a jpeg bytearray.

private static function encodeToJPEG(data:BitmapData, quality:Number = 75):ByteArray {     
    var encoder:JPGEncoder = new JPGEncoder(quality);     
    return encoder.encode(data); 
}

Now encode the ByteArray into Base64.

public static function base64Encode(data:ByteArray):String {     
    var encoder:Base64Encoder = new Base64Encoder();     
    encoder.encodeBytes(data);     
    return encoder.flush(); 
}

Upload the base64 encoded ByteArray to the server.

public static uploadData():void {     
    var url:String = "saveFile.php";     
    var urlRequest:URLRequest = new URLRequest(url);     
    urlRequest.method = URLRequestMethod.POST;     
    var urlLoader:URLLoader = new URLLoader();     
    var urlVariables:URLVariables = new URLVariables();     
    urlVariables.file = jpgEncodedFile;    // as returned from base64Encode()     
    urlLoader.data = urlVariables;     
    urlLoader.load(urlRequest); 
}

The saveFile.php file on the server.