Checks whether a constant with the given name exists

# defined

(PHP 4, PHP 5, PHP 7, PHP 8)

defined — Checks whether a constant with the given name exists

### Description

defined(string `$constant_name`): bool

Checks whether a constant with the given `constant_name` is defined.

This function works also with class constants and enum cases.

> 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().

### Parameters

`constant_name`

  The constant name.

### Return Values

Returns `true` if the named constant given by `constant_name` has been defined, `false` otherwise.

### Examples

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;
}
interface bar {
const test = 'foobar!';
}
class foo {
const test = 'foobar!';
}
var_dump(defined('bar::test')); // bool(true)
var_dump(defined('foo::test')); // bool(true)
?>`

Example #2 Checking Enum Cases (as of PHP 8.1.0)

`<?php
enum Suit
{
case Hearts;
case Diamonds;
case Clubs;
case Spades;
}
var_dump(defined('Suit::Hearts')); // bool(true)
?>`

### See Also

- define() - Defines a named constant
- constant() - Returns the value of a constant
- get_defined_constants() - Returns an associative array with the names of all the constants and their values
- function_exists() - Return true if the given function has been defined
- The section on Constants

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