Synchronizing multiple databases with a Master

[Brad]

I have a problem where there are multiple instances of a mysql database stored on separate local computers that are not always able to connect to the internet. The users edit the database locally using php scripts and then need to upload the new information when an internet connection becomes available. The master database and all the child databases have the same table structure, its just that children may be updating locally at the same time so replication of the master for updating seems like it would overwrite some data.

Is there anyway that I can merge/update/synchronize the data is these databases easily and correctly?

Any help at all would be much appreciated.

Thanks