You can also use VaRest, here is a working example with MySQL and PHP and JSON requests from the VaREST plugin (you can compile it with 4.11.2 and VB 2015 Community edition - needs Component Tools installed) [FREE] VaRest Login System - With PHP and MySQL - Community Content, Tools and Tutorials - Unreal Engine Forums
So i have no idea about security, but theoretically you should be able to use SSL or SSH or a SQL Proxy, and or with salting passwords additionally (bcrypt), API key. One approach could be to use Google Cloud Compute Engine, maybe even with App Engine, and or Cloud SQL or Cloud Datastore, or with a MEAN stack even. Or a container approach with Docker.
I currently try to set up something secure and scalable, i thought to use a standard SQL db for user data, then another (possibly NoSQL, i.e. MongoDB, for item and progress data) …