How to synchronize HTML5 local/webStorage and server-side storage?

Firebase offers this functionality as a service. Another alternative is Parse.

this question is rather similar.

There, 'quick connect' - http://quickconnect.pbworks.com/Using-Enterprise-Synchronization and my personal favorit 'persistenceJS' - https://github.com/zefhemel/persistencejs are recommended.

As soon as I gather personal experience, I'll update this post.

It kind of depends on what you want. There is a great tutorial to get you started at http://hacks.mozilla.org/2010/01/offline-web-applications/ Unless you want to do something complicated, you should not need that much of a framework to do it for you. I guess you could save a few lines of code on the client using jQuery. You basically want to keep in your store a record of when the last change was made to the store, when the last update was to the server, and use send the localStorage variable encoded as JSON in an XHR (or just the bit of the local store you want to send). Then, on the server, you can decode the JSON and do what you want with it, processing it or just using PHP's serialize to dump it to a database.

If you have something particular in mind, do mention it.