This talk shows how I implemented a replication endpoint for the CouchDB replication protocol in PHP. This makes it possible to use about any backend as a synchronization point for Offline-First HTML5 applications powered by tools like PouchDB or TouchDB.
You will also be able to replicate a CouchDB database into a MySQL database or vice-versa. The growing amount of endpoints for this protocol in different languages and environments suggests that the CouchDB replication protocol might evolve as a standard for eventual consistent multi-master replication.