(I'm editing my post to hopefully make it clearer).

I am putting together a web server where I have multiple domains. For this example, I'll keep it simple and say we have two domains (though we could have lots more). test1.com and test2.com are the two domains.

I'm setting this up to test on a virtual machine in VirtualBox. So to go to the domains, you'd put in the browser (as an example) in the host: 192.168.1.4/test1.com to get there.

test1.com directory: /var/www/public_html/test1.com (set to 755, test1.com:test1.com owner:group).
test2.com directory: /var/www/public_html/test2.com (set to 755, test2.com:test2.com owner:group).

Script /var/www/public_html/test1.com/testwrite.php will use file_put_contents to write a test file.

Now if say the testwrite.php writes to /var/www/public_html/test1.com/test.txt the file should also be test1.com:test1.com owner:group.

If testwrite.php writes to /var/www/public_html/test2.com/test.txt then it should not be able to do this. It should instead do nothing or, if php is set to report errors to the browser, say that permission is denied to write the file.

I can't get this to work as I state above. I always get permission denied.

I tried installing suexec-custom and setting it up so that in the /etc/sites-enabled/test1.com.conf file it has:

SuexecUserGroup test1.com test1.com

But even after restarting apache2, it still shows permission denied.

Anyone know how to get this to work?

You have a couple of options. You could use mod_privileges. You could also run separate apache instances within docker containers. I would personally use containers for full separation of services and then proxy pass the containers from the host OS.

This is a test system. I need something that won't really rely on a host in the end because it's a model for a production system. And the production system will not have a lot of memory or space. Things need to be kept as lightweight as possible.

I like the mod_privileges idea and will look into it.

Thank you for the advice.

I looked into mod_privileges and it's not in the repos so I can't even use it. I did try mpm-itk but that didn't work. Got the same results.