many thanks for the advice, that has worked a treat
“Well, may all my unseen fist pounding help others in this scenario:
When Apache is run as a service in Xampp, it requires the libzmq.dll in a different location than when run as a process.
Procmon showed a few places it looks for it (per Procmon, apparently it really was finding php_zmq.dll and therefore providing a misleading error when libzmq.dll was not found), however for Xampp only these two would suffice:
I chose \XAMPP\apache\ because it is just cleaner and easier to see it there.
So for success running as a process or as a service, the following is my resultant set:
Windows Server 2012 r2 Std
php_zmq-1.1.2-5.6-ts-vc11-x86 (…\XAMPP\php\ext\php_zmq.dll & …\XAMPP\libzmq.dll…& …\XAMPP\Apache\libzmq.dll - all files unblocked in properties after download)
\XAMPP\php\php.ini with extension=php_zmq.dll
VC++ Redistributable of 2012 or 2015 (x86 & x64) might play a hand if you have similar struggles, I couldn’t pin enough of that one down to be sure (not going to go uninstall any of them now just to try and find out)
and I’m done with this”
following those steps solved my issue