Debian 9 php5.6 Install

apt install ca-certificates apt-transport-https 
wget -q https://packages.sury.org/php/apt.gpg -O- | apt-key add -
echo "deb https://packages.sury.org/php/ stretch main" | tee /etc/apt/sources.list.d/php.list

apt update
apt install php5.6

apt install php5.6-cli php5.6-common php5.6-curl php5.6-mbstring php5.6-mysql php5.6-xml

https://tecadmin.net/install-php-debian-9-stretch/