I can't believe I have to write this out in this thread of all places but there is a way to do this.

You make sure www-data CANNOT write ANYWHERE on disk. Not /tmp/ Not to /var/www/. No where.

Then make sure your sudoers has nothing weird in it.

Finally disable www-data's ability to run basically anything that isn't required for the operation of your httpd or php-fpm et al.

You then harden it further and further until an RCE does not matter because it's incapable of doing anything.

tldr always treat httpd as untrusted.

Good advice if you're not using the default PHP session settings, but using default settings this means sessions can't be created. At best, that means no-one can log-in. At worst, it completely breaks the site (for those that depend on sessions even for unauthenticated users).

Also: https://stopdisablingselinux.com/ - SELinux can mitigate this.

How do you handle file uploads then? Isn’t that handled by the same process writing the uploaded files to disk?

But then WordPress stops working :)

R/w access to your production database is still a problem, though yes less bad than world read/write.

And also relies on the target being patched against any Linux EoP...