LAMP: Linux/ Apache / MySQL / PHP: Upgrading PHP to 7.1 and enabling extensions

Step 1:

sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install php7.1   # for PHP 7.1

Step 2:

Install extension with the following commands

sudo apt-get install php7.1-gd php7.1-mysql php7.1-dom php7.1-cli php7.1-json php7.1-common php7.1-mbstring php7.1-opcache php7.1-readline