Nginx – Can php-fpm support open_basedir set dynamically for multiple virtual hosts with a single user pool

I am running into a problem where trying to set open_basedir dynamically under nginx (and apache) for php-fpm is "bleeding through" to other vhosts when running php-fpm as user "nobody" (for legacy reasons).

using

fastcgi_param PHP_ADMIN_VALUE "open_basedir=/home/mike/htdocs";

and

fastcgi_param PHP_ADMIN_VALUE "open_basedir=/home/adam/htdocs";

etc. where appropriate for each virtual host (php 5.3.20)

phpinfo shows the correct open_basedir being set, but eventually (rather quickly) I start to get errors thrown where one of the virtual hosts will run into the wrong open_basedir, trying to execute a program for "mike" but having the open_basedir for "adam".

Since fastcgi_param is just basically setting the environment for php-fpm, what I think is happening is the same worker will get it's environment changed while it's still processing a php file.

Somehow servers like litespeed which also use a fastcgi php process (running as nobody) get this right and work correctly with open_basedir being able to be set externally without "bleeding" between virtualhosts.

So I am wondering if there is a trick or workaround I can use to keep running as "nobody" and preserving the environment between vhosts.

I should note I am using unix sockets and not tcp/ip but I don't think that makes a difference.