How do I include a php.ini file in another php.ini file?

I installed Memcached for php and wasn't sure how to make sure that its ini was included in my php.ini file, but as it turns out, it automatically is. You can validate what is being loaded by running php --ini.

php --ini
Configuration File (php.ini) Path: /opt/local/etc/php5
Loaded Configuration File:         (none)
Scan for additional .ini files in: /opt/local/var/db/php5
Additional .ini files parsed:      /opt/local/var/db/php5/memcached.ini

EDIT: My answer was mistaken. This only works in .conf files, which is not the question asked. Better testing showed that it won't work in php.ini files, where include statement is ignored.

I just tested it on Debian 9 (Stretch) with PHP-FPM. From some .conf file, use this syntax:

include=/path/to/special-php.ini

or even

include=/path/to/special-dir-full-of-conf-files/*.conf

as it is used in

/etc/php/7.0/fpm/php-fpm.conf
include=/etc/php/7.0/fpm/pool.d/*.conf

By the way, this will be most useful if you split your settings by topic, and or if you want a set for development and another one for production. Then you could do it the Debian/Apache style like

/etc/php/conf-available/
/etc/php/conf-enabled/

with symliks from the second to the other and an include to that one.