Checking if a constant is empty is bork... 

You cannot

<?php
define('A', '');
define('B', 'B');

if (empty(B)) // syntax error
if (empty(constant('B'))) // fatal error

// so instead, thanks to LawnGnome on IRC, you can cast the constants to boolean (empty string is false)
if (((boolean) A) && ((boolean) B)) 
  // do stuff
?>