I have 2 tables with linked data in a multi-user environment, using the autonumber key from one table as a link. I can write data to one table, use mysql_insert_id to get the ID number, and use this in the next table, but how do I protect against 2 users entering data at the same time?

If you're updating rows in two tables at the same time as part of one "transaction", you'll want to use locking (see commands like LOCK TABLES).  

I know a little about your application, Helen, so I know that the coarseness of table level locking shouldn't cause a problem in your case.   But you might like to be aware that a select that you limit to the first 10 records of a result set, followed by another that returns the second 10 records of a result set, may give duplicated records at the boundary if you've inserted an extra record in the meantime

I did look at table locking, but it seemed a very blunt instrument, and i wondered if there was anything better. However, I suppose the logic of the  insert_id means it would have to be table-level.

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