php 7.1 debian stretch

apt-get install apt-transport-https lsb-release 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" > /etc/apt/sources.list.d/php.list
apt-get update
apt-get install php7.1

dependencies

apt-get install php7.1-bcmath php7.1-intl php7.1-curl php7.1-zip php7.1-gd composer
cd /var/www
composer create-project grumpydictator/firefly-iii --no-dev --prefer-dist firefly-iii 4.7.2.1