I don't think it can be done.... or at least not "real-time" like most drawing applications. Essentially you'd be better off using Flash to allow someone to "draw" their scribble and use PHP to store the image.... whether that's possible, I'm not sure.
But PHP would only be able to draw small bits of lines or blocks from point to point, and wouldn't be fast enough give a good response so you could see what you've drawn instantly like in MS Paint or Fireworks or Photoshop or GIMP.