Overriding $DOCUMENT_ROOT in an Apache Module

I'm working on a custom mass hoting Apache module. I need to be able to set the $DOCUMENT_ROOT value on a per request basis. That is, I'd like to be able to set it in my module before the clients' scripts run (similar to how I use zend_alter_ini_entry to alter ini values). I've tried using Apache's ap_table_set call to define the environment variable, but PHP seems to clobber that and replace it. Any thoughts? Mark