An Apache 2.x Webserver with default designs in the ubuntu/debian databases uses the www-data unix take into account apache2 processes handling web demands. Presuming that apache is serving two different sites (domain1.com and domain2.com), is it feasible for apache to make use of unix user www-data1 when handling demands to domain1.com, and employ unix user www-data2 when handling demands to domain2.com? The motivation would be to isolate the code for every domain title from each other.

Have a look at suEXEC.

suPHP is another nice factor to consider:

"suPHP is really a tool for performing PHP scripts using the permissions of the proprietors. It includes an Apache module (mod_suphp) along with a setuid root binary (suphp) that's known as through the Apache module to alter the uid from the process performing the PHP interpreter."

-http://www.suphp.org/