Most commonly, customizing the PHP configuration of individual apps is what you're trying to do. In a few cases, you may be looking to make global customizations.
Each app has it's own PHP FPM process pool whose default PHP settings can be customized by adding files ending in .conf to the directory:
/etc/phpX.Y-sp/fpm-pools.d/APPNAME.d
To change the default PHP setting for session.gc_maxlifetime to 1 day, create the file:
/etc/phpX.Y-sp/fpm-pools.d/APPNAME.d/settings.conf
with the following contents:
php_value[session.gc_maxlifetime] = 86400
Then, restart PHP with by running the following command as root:
sudo service phpX.Y-fpm-sp restart
If you need to customize the global PHP configuration, you can add configuration files in the following directory:
/etc/phpX.Y-sp/conf.d
Note that files in this directory must end with the file extension .ini rather than .conf.
Next, restart PHP with this command run as root:
sudo service phpX.Y-fpm-sp restart