Debian Stretch PHP multiple versions (5.6.3)
PHP 5.6 is not supported on Debian 9+ (Due to OpenSSL 1.1). Although there are workarounds to replace PHP7 (Not desired as moving forward, PHP7 is the way to go) these does not appear to be a way of running PHP5.6.3 as an optional version- current methods fail at build (Although appear to work if you don't want SSL support).
http://www.linuxcompatible.org/news/story/php_5_6_30_packages_for_debian_9_available.html