@Jacques1 - thanks for the reply. Currently my app is a desktop application written in a number of BASIC languages ranging from BBC Basic up to VB.NET, it's very old legacy code in it.
My tables are flat text files modified by random access (not important). To prevent concurrent write access, I open the file for WRITING and lock it so no other process has access - do my INSERT or REPLACE the close the file. This is all done quickly and as efficiently as possible, no file is ever left open thus locking the entire system. If a file is already open, the next process waits, for 500ms or so for the current process to complete, then it tries again. Works well for users up to 10 or so
As I am new to PHP/MYSQL, I am currently bringing my desktop mindset to this new development world, thus the question: Connect-Update/Replace-Disconnect - do I have to worry about locking.
Your answer about atomic seems to help me a great deal.
I know transactions across mulitple tables is a different matter but I am not there yet!
Thanks
Nigel