Install PHP 5.6 on Debian

Debian 9 come with PHP 7. To install PHP 5.6, run

apt install apt-transport-https lsb-release ca-certificates -y
wget -O /etc/apt/trusted.gpg.d/php.gpg https://packages.sury.org/php/apt.gpg
sh -c 'echo "deb https://packages.sury.org/php/ $(lsb_release -sc) main" > /etc/apt/sources.list.d/php.list'
apt-get update

Install PHP 5.6 with

apt install -y php5.6 php5.6-mysql php5.6-gd php5.6-mbstring php5.6-mcrypt php5.6-zip php5.6-curl php5.6-xml

Enable SimpleXML module

phpenmod -v 5.6 simplexml

Install php-fpm if required

apt install -y php5.6-fpm

See php