You can start a search for php. I am not entirely sure if netrunner core 17.01 is still jessie based. Might be stretch already. So for stretch the package is called php7.0 and is indeed version 7. Version 5 seems to be not available there.
Thanks. I did search and tried different solutions. As well as experimented with Debian 8 (Jessie) in VirtualBox which works well with php 5.6. In stretch not available. Netrunner 17.01 is stretch based.