Black-box testing is a necessity for verifying that applications, as a whole, work. The client uses your application as a black-box. Hence, you should, at least, once, try your application as a black-box prior to letting your client have it.

cURL looks interesting. Do you have anywhere that shows how it's used? The site didn't seem to have stuff about black-box testing WWW interfaces from the way it's displayed on the screen.

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We are the carpenters and bricklayers of the Information Age.

Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.