Anytime I see a function called that's named param() in a question on PM and there's no other info, CGI is a pretty good assumption. This is even stronger given that it's Anonymonk asking the question.

You're right - 'search_string' is most likely a textbox, not a checkbox. However, the same argument still holds.

You're also right in that definedness should be checked for safety. However, truth, not definedness, should be checked for functionality. My reading of the OP's question was that it was a concern of functionality, not safety, of a CGI form s/he was having problems with. You're welcome to disagree, but the question isn't being answered without a few assumptions.

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We are the carpenters and bricklayers of the Information Age.

The idea is a little like C++ templates, except not quite so brain-meltingly complicated. -- TheDamian, Exegesis 6

Please remember that I'm crufty and crochety. All opinions are purely mine and all code is untested, unless otherwise specified.