This is not a PHP problem, but a generic Apache problem. Apache does not clear its environment, and because of that everything that runs under Apache can read it. This includes PHP, mod_perl, mod_python, CGI scripts, etc.

The solution is not to modify PHP, but to start Apache with only the environment it needs. A possible way to do this is to modify your Apache startup script to use the 'env' command to modify the environment when starting httpd.

but just because something CAN read it, doesn't mean it MUST read it, right?

You contradict yourself. First you say "if you don't need to
know, then you don't know" , but then you give all that information to Apache, which does not need to know. And then you shift the problem to PHP, which just gives access to what Apache knows.

PHP just offers an interface to the environment. If you leave a lot of information in the environment, users have access to that information.

It is your responsibility as a sysadmin to make sure the environment does not contain information that should not be available to your users.

PS: to PHP, environment variables are just that: variables. How could PHP know what to give to the users and what not? PHP can't know that, so this is a task for the sysadmin.

Add it to disable_functions in php.ini if you don't want it:

$ cat php.ini
disable_functions = phpinfo

And how does that prevent users from just referencing the variables all by their lonesome outside of phpinfo()?