Thanks to the work of zsalab, Ondřej Surý has released PHP 5.6.30 packages for the upcoming Debian 9 (Stretch) in his DEB.SURY.ORG repository. PHP 5.6 was originally not compatible with the new Debian version because of OpenSSL 1.1

To add this repository please do:

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

  PHP 5.6.30 Packages for Debian 9 available