Fletch advised (wisely!) that you always check that the calls to open succeeded. Is it possible that the file "data/browser.dat" is never actually opened and that's why you never see it amended? In fact, usually the path has to be given in some suitable way relative to the top directory that the web server recognizes, and you may not have that right. (It's happened to me!)
chas
(Update: Checking that open succeeded might be done by writing an error msg to the browser if the open fails.)