Manually install 8.1 if not available

[borpin]

https://github.com/openenergymonitor/EmonScripts/blob/688fce912d5fe8b7a1b9c875338d636350b6ad3e/install/php.sh#L13-L13

A check needs to be done that 8.1 is available or that it failed to install.

If not, install from PPA.

sudo apt install software-properties-common
sudo add-apt-repository ppa:ondrej/php
sudo apt update

[borpin]

175 installs PHP8.1 unless it is an ARMv6 architecture