The app enables employees at various locations to stay up to date with the same information... kind of like a CRM accessed from multiple locations. The difference is that they still need to be able to enter information if they have a connectivity lapse, whereas office based workers would just have a tea break until the connection was restored.
Due to the conditions the app is going to be running in, I'm able to specify a minimum browser requirement, so am happy to rely on an HTML5 feature such as local storage. But you're quite right, there does need to be a way to sync it all together once the browser is back online and is able to send its locally stored data. That can be dealt with server-side in PHP.
I've done a little more research on local storage and have found that it's persistent between browser sessions. This would make it perfect for my needs, so the main headache is now the server-side problem of ensuring it's all in sync.
I'm loving HTML5 already (assuming this all works!).