Unfortunately, PHP does handle a couple of request parallel. As I don't know exactly what these data are for, I can suppose two ideas for updating the database:

1. Lock it!
Add a new field called...

seems to me as if you have an empty line at the beginning of your code. Make sure that the php-tag "<?" starts directly at the beginning of your file, that all code that you include before don't echo...

When accessing a database in PHP you first don't have to bother of whom the data might belong to. You can access all of them and sort out which of them is available for the user and which of them...