multisite, php evaluator and open_basedir

Hi, hope this is not the wrong place to ask...

I read through most of the multisite relevant info here and elsewhere, there are many interesting posts that help a lot getting familiar with that very interesting feature of Drupal.

However, I am stuck at the fact, that an Administrator of a site will always be able to post php as content for a node - I can not find a configuration item that would prevent this.

So in a multsite setup the Adminstrators of all sites will always be able to include the config file for all sites and e.g. access the databases of other sites, at least with the recommended directory structure that comes with drupal.

I found some ideas here that could prevent that when combined with some per site open_basedir restrictions, but at the moment I am not sure, if that will lead to other problems...

To sum up what I want: give people their own drupal site - but NOT to be able to hack into other sites on the same server.

Of course another approach would be to prohibit posting of php code as node content for Administrators - how can this be done?

What do you think about that problem? Is it a problem at all or am I missing something?

Thank you very much for your attention!