(PHP 4, PHP 5)
defined — Checks whether a given named constant exists
$name
   )Checks whether the given constant exists and is defined.
Note:
If you want to see if a variable exists, use isset() as defined() only applies to constants. If you want to see if a function exists, use function_exists().
nameThe constant name.
   Returns TRUE if the named constant given by name
   has been defined, FALSE otherwise.
  
Example #1 Checking Constants
<?php
/* Note the use of quotes, this is important.  This example is checking
 * if the string 'TEST' is the name of a constant named TEST */
if (defined('TEST')) {
    echo TEST;
}
?>