How to secure site?

Hello! I have an Apache web-server with PHP-4.3.2 installed. Directory structure of my DocRoot is as follows: www/

    docs/
    tm/
    tts/
    cache/
    lib/
    index.html

docs, tm, tts are directories of projects, published through ftp by different people, lib - common directory, containing php-scripts, cache - common directory, where scripts of all projects must be able to create temporary files. I want my projects not to be able to read scripts of each other, BUT use common dirs. open_base_dir in each directory of docs, tm, tts seems to be be a good idea. BUT, in this case projects scripts won't be able to use cache and lib. Other way - I can turn on safe_mode and include www/lib in safe_mode_include_dir. In this case projects will be able to include scripts from lib, BUT won't be able to create files in cache because scripts and cache have different owners. So, can you tell me some decision to solve my problem? -- Andrew