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 install -y lsb-release ca-certificates apt-transport-https software-properties-common
sudo 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 -y php7.1 php7.1-cli php7.1-common php7.1-json php7.1-opcache php7.1-mysql php7.1-mbstring php7.1-mcrypt php7.1-zip php7.1-fpm
php -v
php -m
sudo systemctl status php7.1-fpm