# yum install gcc plesk-php56-devel make zlib-devel
# /opt/plesk/php/5.6/bin/pecl install memcache
# echo "extension=memcache.so" > /opt/plesk/php/5.6/etc/php.d/memcache.ini
# plesk bin php_handler --reread
Check that module is loaded:
# /opt/plesk/php/5.6/bin/php -i | grep "memcache support"
memcache support => enabled