restricting filesystem access

Hi, I am a new php user and I have a question, for which I couldn't find any answer. I'd like to restrict php code to access the filesystem. I'd like to have only one directory where the php code can write, create or read files, and an other directory hierarchy where the php codes present. I need this to avoid php code to rewrite other php code in case of bug and/or an attack. I already tried the open_basedir directive, but it couldn't work because in this case the executed php have to be in the accessable directory hierarchy. Thanks, Gabor