$TileData is still a string instead of a number, so it doesn't work with the bitwise operators.

Well, of course it doesn't since my suggestions related to the file handling aspects of your code only (as I thought I made clear in my original response) because BrowserUk had already answered the bitwise operator aspects of your problem. Please re-read his answer and study the documentation for pack and unpack then show us your code and explain precisely why it doesn't do what you want it to.