title: Why do undefined constants evaluate to true?
body: <p>Stupid question - I'm surprised this one has bitten me.  Why do undefined constants in PHP evaluate to true?</p>
  
  <p>Test case:</p>
  
  <pre><code>&lt;?php
      if(WHATEVER_THIS_ISNT_DEFINED)
          echo 'Huh?';
  ?&gt;
  </code></pre>
  
  <p>The above example prints 'Huh?'</p>
  
  <p>Thanks so much for your help! :)</p>