(PECL uopz 1, PECL uopz 2, PECL uopz 5, PECL uopz 6)
uopz_undefine — Undefine a constant
$constant
   ) : bool$class
   , string $constant
   ) : boolRemoves the constant at runtime
class
 The name of the class containing constant
     
constantThe name of an existing constant
   Returns TRUE on success or FALSE on failure.
  
Example #1 uopz_undefine() example
<?php
define("MY", true);
uopz_undefine("MY");
var_dump(defined("MY"));
?>
The above example will output:
bool(false)