Will simultaneous access defeat this strategy?

[Robert Oschler]

I want to have a master table and a detail table. I want the master table
record to be created only if one does not exist already for a given account
ID.

Here is my concern:

Let's say I have several different web pages being viewed by several
different web surfers. The viewing of the web pages triggers the same PHP
code that attempts to locate a desired master record, auto-creating it if it
does not exists.

Could I end up with several copies of master records with the same account
ID, since the multiple running instances of the PHP code did not find the
master record when they went to look for it? This would break the strategy
since the strategy is dependent on there being one and only master record
for a particular ID.

If so, what is the proper coding strategy for enforcing this strategy?
Locking, semaphores, etc? Any samples you can point me to would be more
than welcome.

thx

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Robert Oschler