Here’s how I managed to install the memcached module for php 7.0 running on a server running CentOS with DirectAdmin.

Install the remi repo and install the latest version of memcached and libmemcached

yum -y install http://rpms.remirepo.net/enterprise/remi-release-$(awk -F 'release ' '{print $2}' /etc/system-release| head -c 1).rpm
yum --enablerepo remi install memcached libmemcached-last libmemcached-last-devel git

Now configure and install the memcached extension:

cd ~
git clone https://github.com/php-memcached-dev/php-memcached.git
cd php-memcached
git checkout php7
phpize
./configure --disable-memcached-sasl --with-php-config=/usr/local/php70/bin/php-config
make
sudo make install

Add the extension to the php config:

echo "extension=memcached.so" | sudo tee /usr/local/php70/lib/php.conf.d/60-memcached.ini

And finally, restart php-fpm:

service php-fpm70 restart

Or if you run mod_php:

service httpd restart