How to check if a defined constant exists in PHP?

So I'm using a PHP framework called fuelphp, and I have this page that is 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?
I want to check for the the fuelphp framework file locations.

These are the constants I need to check for (actually, I only have to check one of them):

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';

edit:
I realized that these aren't variables they are constants...

First, these are not variables, but constants.

And you can check their existence by using the defined() function :

bool defined ( string $name )

Checks whether the given constant exists and is defined.

Use defined() function, for example:

if (defined('VAR_NAME')) {
    // Something
}

Check using defined('CONSTANT') function.

An example from the manual:

<?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;
}
?>

here's a cooler & more concise way to do it:

defined('CONSTANT') or define('CONSTANT', 'SomeDefaultValue');

credit: daniel at neville dot tk https://www.php.net/manual/en/function.defined.php#84439

I take it you mean CONSTANTS not variables! the function is defined();

see here: defined

How to check if a defined constant exists in PHP?

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