title: If isset for constants, but not defined?
body: <p>If I set a constant to = <code>''</code>,<br>
  How to I check if constant has something inside ?<br>
  (ie see if it is set to something other than the empty string.)</p>
  
  <p><code>defined()</code> does not do what I want, because it is already defined (as <code>''</code>).<br>
  <code>isset()</code> does not work with constants.</p>
  
  <p>Is there any simple way ?</p>