How to remove php 5.6

Package named php5 or php5.6 is just a meta-package that pulls one of the available web SAPIs (apache2, fpm or cgi). The easiest way how to remove full stack of packages is to remove php5-common and/or php5.6-common package as everything else depends on it, e.g.:

apt-get purge php5-common

The other option would be to use wildcard support in apt-get and do:

apt-get purge 'php5*'