How to check if a defined constant exists in PHP

Question

I am using fuelphp a PHP framework and I have an  HTML file, so I can't use PHP in it. I have another file that has a top bar in it, which my HTML file will call through ajax.

How do I check if a constant exists in PHP?

These are the constants I need to check for:

define('DOCROOT', __DIR__.DIRECTORY_SEPARATOR);
    define('APPPATH', realpath(__DIR__.'/fuel/app/').DIRECTORY_SEPARATOR);
    define('PKGPATH', realpath(__DIR__.'/fuel/packages/').DIRECTORY_SEPARATOR);
    define('COREPATH', realpath(__DIR__.'/fuel/core/').DIRECTORY_SEPARATOR);                    
    require APPPATH.'bootstrap.php';

Can someone help me with this?