Installing PHP 8.1 on Ubuntu 20.04

php80.sh

# PHP 8.1
sudo apt install software-properties-common
sudo add-apt-repository ppa:ondrej/php
sudo apt-get -y install php8.1-cli 

# Composer
cd ~
php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
php composer-setup.php
sudo mv composer.phar /usr/local/bin/composer
rm composer-setup.php

# Postgresql
sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt/ `lsb_release -cs`-pgdg main" >> /etc/apt/sources.list.d/pgdg.list'
wget -q https://www.postgresql.org/media/keys/ACCC4CF8.asc -O - | sudo apt-key add -
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install postgresql postgresql-contrib libpq-dev pgadmin3
sudo apt-get -y install php8.1-pgsql

# MySQL
sudo apt-get -y install mysql-server

# Additional extensions
sudo apt -y install php8.1-fpm php8.1-curl php8.1-zip php8.1-mbstring php8.1-xml php8.1-gd php8.1-mysql

# Additional Softwares
sudo apt -y install nodejs npm zip unzip imagemagick php-imagick

This doesn't seem to work on 18.10. At least not anymore.

This doesn't seem to work on 18.10. At least not anymore.

Did you add the repository and update?
sudo add-apt-repository ppa:ondrej/php
sudo apt-get -y update

I just configured on a Ubuntu 18.04 on Azure.