Apparently Memcache and Memcached are old technologies that are not fully supported by PHP 7 - see links on https://docs.moodle.org/36/en/Talk:Caching The Moodle Caching documentation needs to be updated to remove the references to Memcache and Memcached and clearly describe what the modern alternatives are. And it needs to clearly describe what needs to be done and implemented to get caching in Moodle setup correctly using more modern caching technologies like, I presume, Redis and MongoDB. This is especially relevant to sysadmins who need to configure large Moodle installations with multiple load-balanced web server front-ends. The current documentation is difficult to follow and has huge gaps, especially for people still new to setting up Moodle sites.