[jijji]

PHP-FPM is very easy to debug. It's as simple as setting it up to use a listening socket in www.conf (located in i.e. /etc/php/8.1/fpm/pool.d) and then running a packet sniffer (i.e. ngrep) to listen on that socket, all messages back and forth are visible at that point.

[lapser]

I don't know if I'd call usage of a packet sniffer "easy to debug". Seems like it's reasonable to expect some debug option that is easy to activate in dev environments that will give you the relevant information.

[jijji]

The back and forth communication is all that you care about with php-fpm, and rarely do you need to actually see it, unless you have php-fpm configured wrong for instance. The majority of debugging a php application means turning on debugging (i.e. ini_set 'display_errors' On and error_reporting('E_ALL')) and tailing a /var/log/nginx log file looking at what happened in your application.