Pi-hole on Debian Bullseye

It would. PHP is just used for the web interface, and this is where you have problems.

Interesting that Bullseye is not reporting a version yet. They may hold this until final release. With Raspbian Buster, the OS is reported as this:

PRETTY_NAME="Raspbian GNU/Linux 10 (buster)"
NAME="Raspbian GNU/Linux"
VERSION_ID="10"
VERSION="10 (buster)"
VERSION_CODENAME=buster
ID=raspbian
ID_LIKE=debian

With Armbian Buster, it's:

PRETTY_NAME="Armbian 20.11.6 Buster"
NAME="Debian GNU/Linux"
VERSION_ID="10"
VERSION="10 (buster)"
VERSION_CODENAME=buster
ID=debian