You can change the default php.ini file in: /usr/local/phpXX/lib/php.ini
and the restart the according php version. If you have set php.ini to yes in custombuild, it might be overwritten on an update, so disable this.
Or you can create a custom .ini file in the php.conf.d directory for example: /usr/local/phpXX/lib/php.conf.d/11-custom.ini
Then in that custom.ini file place your custom options there.
Als restart php-fpmXX afterwards.