Install php8.1 ubuntu 20.04

Update system

sudo apt update
sudo apt upgrade

Add repository

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

Install php8.1

sudo apt install php8.1

Install features

sudo apt install php8.1-common php8.1-mysql php8.1-xml php8.1-xmlrpc php8.1-curl php8.1-gd php8.1-imagick php8.1-cli php8.1-dev php8.1-imap php8.1-mbstring php8.1-opcache php8.1-soap php8.1-zip php8.1-redis php8.1-intl -y

Enable php8.1 disable php7.4

sudo a2enmod php8.1
sudo a2dismod php7.4

Restart apache

sudo service apache2 restart

running multiple versions: How To Install Multiple PHP Versions with Apache on Ubuntu 22.04/20.04/18.04 (tecadmin.net)