By running
rcctl -d start php72_fpm
(note the -d), you will start the service without redirecting the output from the rc.d script to /dev/null. This would allow you to see what's happening.
This is me trying to start php72_fpm from freshly installed php-7.2.10 installation.
$ doas rcctl -d start php72_fpm
doing _rc_parse_conf
doing _rc_quirks
php72_fpm_flags empty, using default ><
doing _rc_parse_conf /var/run/rc.d/php72_fpm
doing _rc_quirks
doing rc_check
php72_fpm
doing rc_start
doing _rc_wait start
doing rc_check
[04-Nov-2018 23:44:31] WARNING: Nothing matches the include pattern '/etc/php-fpm.d/*.conf' from /etc/php-fpm.conf
at line 125.
[04-Nov-2018 23:44:31] ERROR: No pool defined. at least one pool section must be specified in config file
[04-Nov-2018 23:44:31] ERROR: failed to post process the configuration
[04-Nov-2018 23:44:31] ERROR: FPM initialization failed
doing _rc_rm_runfile
(failed)
The issue here is that the configuration is missing.
Make sure that you copy the supplied example configuration file from /usr/local/share/examples/php-7.2 into /etc/php-fpm.d (and modify it to fit your needs).
See also: