Is this code thread safe?

I'm pretty sure this code is thread safe, but I just want to be 100% sure. I have a class called ViewHelper with a static function called is_signed_in. All it does is check the session for the existence of a particular variable, and if it's there, it returns true, otherwise false. Since the function is static, is there any way this code is not thread safe? Is there any way I could be checking someone else's session if the requests are concurrent? Thanks, Christian