Securing php

[Josh]

I am setting up a apache + php + mysql which will have shared hosts on it,

How do I make it so that php cant read or execute files outside of 
/home/user/web for example.

I know that there is safe_mode, but there must be a more definite way, 
and safe_mode is gone in php6 anyway.

Is there some sort of acl/mac thing I can use, or something magical I 
have not heard of yet? At the moment I am running php as a cgi with 
fastcgi + suexec, so I can run each vhost under a different uid/gid.

Any ideas? Main goal: PHP instances unable to access out side of 
relevant vhost document roots.

Thanks,
    Josh