Drupal Memcache
Had to install as follows (need to be php7.x):
1pecl install memcached
Dependencies may need to be installed
1# yum groupinstall "Development tools"
2# yum install zlib zlib-devel
3# yum install php php-devel php-pear
4echo "extension=memcache.so" >> /etc/php.d/memcache.ini
See https://tecadmin.net/install-memcached-with-pecl-memcache-on-centos-rhel/
For Ubuntu 16.x (with php5.6.x)
1apt-get install memcached
2apt-get install php-memcache