Currently WinCairo uses PHP 7.1. GTK seems to use both 5.x and 7.x. Apple looks like sticking around PHP 5.x. The support of PHP 5.x and 7.0 is going to end by Jan 2019, six months from now. http://php.net/supported-versions.php
Noting what our actual solution to this is: We're replacing PHP tests with Python.