The php stack I am using for blogs and other hosted php websites is from Remi. I am on PHP version 5.6. The memcached plugin was enabled and I never checked its status. I only found out that the memcached is not enabled at all on my websites.
OK I figured this will be a simple task. It is. There are a few things that you need to be cautious about if you are using Remi as php base. The modules will not be loaded magically.
1. Copy or link the module to your system library, you need to do the same for 3 files for PHP 5.x:
for i in igbinary.so memcached.so msgpack.so do ln -s /opt/remi/php56/root/usr/lib64/php/modules/$i /usr/lib64/php/modules/$i done
For PHP 7 you do not need the igbinary file.
2. Add the following modules to /etc/php.d directory using ordering, each of the file contains only a line of
extension = xxx.so
extension = msgpack.so
extension = igbinary.so
extension = memcached.so
3. Restart php-fpm
Depend on your configuration, you might not be using php-fpm. Restart it so that the changes can apply to your website.
When you go to Tools -> Memcached in WordPress, you shall see this line:
Using the PHP Memcached class to interact with Memcached
It proves that you are using Memcached!