array_key_exists

Hey /prog/,

I'm working on an assignment in my web developing class, and I can't for the life of me figure out how to check if a fucking session cookie has a value in it.  We're supposed to use the array_key_exists function, but all google tells me is how to check for a specific value.  I don't care about a specific value, I just need to check if the cookie has ANYFUCKINGTHING in it. 

I tried array_key_exists("", $var) but no go.  I'm missing something I think. 

Any help? <3

It is a cookie, but I just need to check for any value.  The cookie is being set based on user input, so the value could be anything.