at /usr/local/directadmin/custombuild/php_extensions.conf
set opcache=yes
then run:
/usr/local/directadmin/custombuild/build update
/usr/local/directadmin/custombuild/build opcache
---
memcached must be installed manually,
daemon can be installed by first part ot this manual: https://www.tecmint.com/install-memcached-on-centos-8/
how to install it additionaly as PHP-extension - use google, there are different ways.