Starting with a fairly stock Debian 10 PHP/Apache2 install which defaults to the
mpm_prefork module and
mod_php. It seems to be safe to install
php-fpm in place, as it won't clobber the existing PHP and Apache configuration:
sudo apt install php-fpm
See the following comments after the install:
Creating config file /etc/php/7.3/fpm/php.ini with new version NOTICE: Not enabling PHP 7.3 FPM by default. NOTICE: To enable PHP 7.3 FPM in Apache2 do: NOTICE: a2enmod proxy_fcgi setenvif NOTICE: a2enconf php7.3-fpm NOTICE: You are seeing this message because you have apache2 package installed.
So now we just need to enable the additional apache modules to support
a2enmod proxy_fcgi setenvif a2enconf php7.3-fpm
mod_php config/module is safe to disable; it's config is located in
/etc/apache2/mods-enabled/php7.3.conf; disable the module with:
sudo a2dismod php7.3
At this point once the apache process is restarted, the
php-fpm configuration will take over.
Simple as disabling the prefork module, and enabling the event module as follows:
sudo a2dismod mpm_prefork sudo a2enmod mpm_event
Restart apache and voila.
The configuration file for the
mpm_event module is located at
/etc/apache2/mods-enabled/mpm_event.conf. Good luck with that configuration; here are some (hopefully up-to-date and helpful) tips.