I sometimes manually install a Debian package, whilst trying to figure out dependencies for something else, and then discover I should have installed a different "parent" package that will pull it in.

In the past I have left this situation as-is or, if I'm feeling particularly finicky removing the manually installed package and then installing the parent which re-installs it as an automatic dependency.

Anyway, there's a proper way to set a package as automatic (so $ apt-get autoremove will remove it if it's no longer required) and I finally looked it up. Here it is in all it's simplistic glory: 

$ apt-mark auto php7.0

This Blog post is almost entirely an aide-memoir to myself!