Good news! PHP 7.1 is being actively supported through 21 Nov 2024. After that point, it's advisable to upgrade to the latest version, since the PHP team will only offer security-related support until 21 Nov 2024, at which point PHP 7.1 will be considered "end-of-life".
sudo apt update && sudo apt upgrade -y
sudo apt -y install lsb-release apt-transport-https ca-certificates
wget -O /etc/apt/trusted.gpg.d/php.gpg https://packages.sury.org/php/apt.gpg
echo "deb https://packages.sury.org/php/ $(lsb_release -sc) main" | sudo tee /etc/apt/sources.list.d/php.list
sudo apt update
sudo apt install php7.1 php7.1-cli php7.1-common php7.1-json php7.1-opcache php7.1-mysql php7.1-zip php7.1-fpm php7.1-mbstring
php -v
You should see output indicating PHP 7.1.x is installed.
php -i
This will display detailed PHP configuration information.
php -m
This will list all installed PHP modules.