An Apache 2.x Webserver with default configurations from the ubuntu/debian repositories will use the www-data unix account for apache2 processes handling web requests. Assuming that apache is serving two different sites (domain1.com and domain2.com), is it possible for apache to use unix user www-data1 when handling requests to domain1.com, and use unix user www-data2 when handling requests to domain2.com? The motivation is to isolate the code for each domain name from one another.
Take a look at suEXEC.