When bob will log in at that time you might be making remotecall to check credentials. Now you have userid right so pass
the same userid in other remotecall where you will get data related to Bob and then show it in datagrid accordingly.
Hope you got my point!!!
The work of filtering the data for the particular user should be done by PHP/MySQL or whatever other server-side solution you may be using. It's best to have your have your Flex app be ignorant of details such as this. On the Flex end, keep it simple by assuming that the data provided is correct and go straight to plugging it in (of course with some exception handling).
You probably don't want to be passing that user id around in the open. Make use of PHP sessions.
If the code I sent you needs adjusting let me know via email