This read only variable returns the unique identifier for the game you have created. You can use this if you need a unique file name, or anything else that needs something to identify your game only. This can be set in the Game Options.
ini_write_real("Scores","0",score + game_id);
The above code performs a very basic encryption on the score by adding the game_id to it before saving. On reading it back into the game you would deduct the game_id to get the correct value again.