Is your script designed for MySQL alone? Does it require MySQL to function at all? Or is it designed for `a database' and can run under MySQL, PostgreSQL, or something else?
If you've designed it strictly around MySQL then you've really made a commercial product that requires the engine, so you need to buy a license.
If, however, you can support any database with a base level of functionality (and you don't rely on anything that only MySQL supports) you probably DO NOT have to pay.
It also depends on what you're doing for the client. Are you tying into a database you expect to already exist by the time your software is purchased? Then you again probably don't have to pay. Or do you plan to distribute MySQL or give them detailed instructions on installing it with your software? Then you probably do.
One thing I'd like to suggest, folks. Look, development won't last forever if nobody pays. I agree that buying a copy for every software user is prohibitive compared to other products available but would it not be courtesy for commercial developers to register their OWN copy that they develop with?
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