The phpMyAdmin package includes various features from PHP 5.6, causing problems for an existing installation of PHP 7.n. It should be PHP-version agnostic.
I'm afraid that this is a limitation of the ports tree and not something that can be solved through the phpMyAdmin port. You can add eg. DEFAULT_VERSIONS+= php=7.0 to your make.conf and build from ports if you want to run php7.
(In reply to Matthew Seaman from comment #1) I'm sure it "works as intended", Matthew. It's the *intention* that's the problem. v4.7 (and 4.6 before it) is happy with any php version from 5.6 through 7.1. So it should not be packaged with any particular version of PHP. It should expect to find PHP already installed and waiting when it gets there. Telling me I can build my own copy seems to miss the point. If we often have to work around the choices made by the ports team, what does that say about those choices?