How do I copy a Sprite instance into a BitmapData instance?

I apologize if this is covered elsewhere: I just don’t even know what terms to search on to find what I’m looking for…

I have a sprite that receives various user-drawn lines on it during runtime. I would like to be able to copy this Sprite into a BitmapData instance, then send the pixels from the BitmapData instance to a PHP script. I think I’ll be all right with the PHP end of things, but I just don’t know how to copy my Sprite into a BitmapData.

(If there were a way to just use BitmapData and use the moveTo, lineTo, mouseX, mouseY, etc, methods, then I’d do that, but I don’t even know if that’s possible…Obviously, I’m not as well-versed in AS3 as I’d like to be.)

Thanks in advance for any help!