vb6 to php to mysql and back

Take a look at ADO Recordsets they can save and load recordsets as XML files.

You could write a dll that runs on the server. You call one of its routines and it grabs the data from mysql into an ADO recordset and then saves the data to an xml file. Then your vb6 program grabs the xml file from the server and loads it into its own ADO Recordset and the reverse when you need to send data back. Of course sending data back for updates would create potential problems with mutlipule users trying to update the same records. So you would need a way to decide which records update. Or use replication where you have control down to the field level.