One thing I don't like is that $country is first used as temporary storage for the result of the function call, then used to mean something almost completely different. Variables are supposed to be variable, but this feels like a case where separate variables would map the business rules better.

Oh, and what if $card->country is "0"? I'm guessing you would want that translated to "" or "[0]".

Update: I wasn't going to propose an alternative, but ended up doing so as a response to another respondent: Re^2: How Much Is Too Much (on one line of code)?.