PHP reference

defined

Checks whether a constant exists.

Syntax

defined(string $constant_name): bool

Parameter	Type	Description
$constant_name	string	The constant name as a string.

bool`true` if the constant is defined.

A default value for a constant:

<?php
 
if (!defined('ITEMS_PER_PAGE')) {
    define('ITEMS_PER_PAGE', 20);
}
 
var_dump(defined('ITEMS_PER_PAGE'), defined('NO_SUCH_CONSTANT'));

Output:

bool(true)
bool(false)

Pass the name as a quoted string. Using a non-existent constant without the check raises an error.

Constants — Constants in PHP — const and define, naming rules, built-in and magic constants.