How can I fix this error?

[dvarsam]

Hello all!

I have the following problem:

I have created the following list:

When I select the first option - i.e. "--Select One--" (or default value), I get the following error:

I want to write some code to tell my PHP to launch a popup window, warning the user that he must select a choice instead of the using the default "--Select One--" option?
How do I do that?
What code should I write?

Thanks.

P.S.> Sorry, but my PHP book does NOT provide a solution to this...