First of all, it isn't unsecure, as the data isn't being used except to see it was provided. Sure anyone could pass a querystring variable, but I can show you site after site after site that uses querystrings to show you what you asked for. Think of all the pretty URLs. My example isn't any different.
I can't believe that because 90% of requests you see today browsing this forum do this very thing (go ahead, look at the URL). Look at the Logout URL of this forum too while you are at it.
The OP poster asked how you can read items coming from a LINK, not a FORM. I answered that respectively. Not everything can be a form, everything shouldn't be a form. Granted, maybe we read his request two different ways?
I read it as "How can I use isset when working with a LINK?" There are two ways, the most common is passing a variable and using isset on the $GET request. The second is reading the HTTPREFERER (not always available).
Lesson to Learn: Next time on an ambiguous question, maybe we can identify how we interpreted the question along with our answer to avoid this confusion.