How to synchronize multiple local sites with a server?

Question

PHP

There is a certain website using PHP and MySQL.
There are several Windows 7 machines on which this site should work, but the Internet on these machines is not constant or stable. Accordingly, it is necessary that local copies of this site work offline and, preferably, be automatically updated from the server.
Making the site work locally is not a question, but the question is how to update it regularly? That is, update MySQL and changed/added files.

Answer 1

One-way sync - easy.
Though backups of a DB do in the center and recover on local copies.
Though one-way replication master-slave.
But what if they change something on the locals.
It won't be easy to sync.