Why can't upgrade to php7.3 on Ubuntu 16?

Question

PHP

sudo apt-get install software-properties-common
sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
sudo apt-get install -y php7.3

Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package php7.3
E: Couldn't find any package by glob 'php7.3'
E: Couldn't find any package by regex 'php7.3'

apt-get install software-properties-common installed

software-properties-common is already the newest version (0.96.20.10).

Answer 1

Because the 16th version of ubuntu xenial is outdated by two lts releases, and in a month there will already be three . There are no assemblies for xenial in
the specified ppa , so be careful: https://launchpad.net/~sergey-dryabzhinsky ps , if you can’t update the system in any way, virtualize what you can, either with full-fledged virtual machines or using lxc or docker

Answer 2

He has only 7.4 in his turnip and that one until 11/28/21.
What is written on his gith at the beginning, what according to the versions, which in support poke here