@mfechner correct, you had a stale WITH_MPM=event in your env and so the heuristic thought to find extensions dependencies in the zts dir, because of threaded apache mpm.

The main question is if it is really needed at all. I never saw a use-case for the zts/debug handling. While i can understand the idea you explained, it looks like these cases will only occur when building the code by yourself AND switching such basic options afterward. And i think with pkg becoming the main source of installation this scenarios are even more unlikely. So i personally would try to remove it all, but the last attempt failed badly :D

In D26848#599504, @tz wrote:

Are you sure it's your basic setting? The ports failing because of the ZTS option. And this option is broken since 7.0 or even earlier. Same for DEBUG option. See for example here: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=245034

So this is expected ;)

The php.mk part looks good to me.