Which versions of PHP do we need this to work on?

[mike42]

We will need to introduce tests to avoid breaking supported PHP versions. The more versions we support, the more locked down we are on which dependencies, language features etc we can use.

At the time of writing, we have 5.6 and 7.0 being tested already, which does not include any versions outside of security update coverage.

Are we happy to adopt a practice of dropping compatibility with unsupported versions of PHP at each major release?