Thanks for the reply SKaero.
Forgive the newbieness of this question, but ...
Given I'm running this on an macOS 10.11 box with Server.app v5.2, I don't think that I can easily upgrade PHP or any of its modules. Is there any way to install a 2md instance of PHP, and direct RoundCube to use that, rather that the built-in version?
TIA
Ian