There were issues with Mosquitto 1.6.2
Feed from one input stops updating, others feeds continue to work
due to the older images using the Mosquitto repos rather than Debian’s, we are currently using 1.4.10, Buster uses 1.5.7, i seem to recall that when the raspbian repo was at 1.5.? users (Brian?) were rolling back to the 1.4.10 due to some other errors, where between 1.4.10 and 1.6.2 things fall over i don’t know.
Yeah, initial php7 support came in at the end of 2016, little has happened since then, only 2 commits since the end of 2017!
A couple of those links above do use 7.3, but the issues were with the Mosquitto version not so much the PHP version AFAIK.
The extension hasn’t been updated since March 2017 on PECL, php 7.3 didn’t release until 2018 so if it is compatible it’s more chance than anything.
I wasn’t suggesting dropping MQTT support, just changing the way it’s done. You maybe flogging a dead horse with support for MQTT in PHP, even if PHP7.3 does work with Mosquitto 1.5.7, what happens when we reach Mosquitto 1.6.2 (already known to be an issue)? or php7.4, scheduled to be released on November 21, 2019?