Hi Adam,
For Moodle 3.5 PHP 7.x is required. When we first tested Moodle version 3.4, Memcached wasn't supported anymore in PHP 7.1.8, so now we use the Redis server as caching server in our Moodle test environment. Here you can find more information how we configured Redis on our server:
https://moodle.org/mod/forum/discuss.php?d=366556
Installing and upgrading help
Memcahed and moodle 3.5
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