The isset() calls are a little redundant, if you only want to know if the variable has a (non-empty) value. empty() is a language construct (not a function), that just like isset(), won't throw error notices on undefined variables/indices.
'empty' on a unset variable should throw a warning; or at least it used to. Hmm... did they finally get rid of that?
According to the manual: http://www.php.net/manual/en/function.empty.php Not sure if it has always been like that, though.