(PHP 3, PHP 4 >= 4.0.0)
defined -- 
     Checks whether a given named constant exists
    
Description
bool 
defined ( string name)
     Returns TRUE if the named constant given by
     name has been defined, 
     FALSE otherwise.
      
Example 1. Checking Constants <?php
if (defined("CONSTANT")){ // Note that it should be quoted
    echo CONSTANT; //
    }
?> |  
  | 
    
     See also define(),
     constant(),
     get_defined_constants() and the section on
     Constants.