How to check if a defined constant exists in PHP?

Question

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...

@FrankPresenciaFandos Yes I have done that, but all the tries I did, got me no where. — ryanc1256, Jan 20 '13 at 21:11
@ryanc1256 concerning the answer downvotes, these indicate an incorrect / harmful answer. — John Dvorak, Jan 20 '13 at 21:15
@EricMORAND, I downvoted the question for thinking he didn't use google. Then I realized it was a problem with the name, so I edited the question and didn't vote. — Francisco Presencia, Jan 20 '13 at 21:15
@ryanc1256 , apparently my edit disappears if you edit it in the meantime. Just change every "variable" in the question for "constant" — Francisco Presencia, Jan 20 '13 at 21:17

score 86 · Accepted Answer · edited Feb 22 '18 at 15:35

86

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.

edited Feb 22 '18 at 15:35

kenorb

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answered Jan 20 '13 at 21:08

Eric MORAND

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score 83 · Answer 2 · edited Feb 22 '18 at 15:36

83

Use defined() function, for example:

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

edited Feb 22 '18 at 15:36

kenorb

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answered Jan 20 '13 at 21:07

Tom Walters

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17

And to save a few extra minutes of debugging, make sure the `VAR_NAME` is included in quotes, as shown in this answer and in the docs. – Sablefoste Apr 19 '14 at 14:39
1

Adding the quotes, so simple but so simple to overlook! – Aaron Belchamber Jul 24 '18 at 12:51

score 41 · Answer 3 · edited Feb 22 '18 at 15:36

41

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;
}
?>

edited Feb 22 '18 at 15:36

kenorb

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answered Jan 20 '13 at 21:07

Niklas

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Incorrect; OP's "variables" weren't variables - visible from the code from the first revision, clarified in text later on. – John Dvorak Jan 20 '13 at 21:22
You first questioned about how to check if a variable exist .. and then you changed it to a constant. ... It would be polite if you don't vote down my post ... – Niklas Jan 20 '13 at 21:27
I didn't - I'm not the asker. Note that votes are about correctness as well as effort. Note that deleting answers is free and reclaims your points, and you would have to do it anyways (or edit until it's correct). – John Dvorak Jan 20 '13 at 21:31
Note that link-only answers are not welcome here anyways (also note you've linked to the german translation by accident) – John Dvorak Jan 20 '13 at 21:32

score 23 · Answer 4 · edited Jul 14 '19 at 19:47

23

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

edited Jul 14 '19 at 19:47

SherylHohman

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answered Jun 11 '16 at 12:49

Svetoslav Marinov

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score 5 · Answer 5 · answered Jan 20 '13 at 21:08

5

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

see here: defined

answered Jan 20 '13 at 21:08

Zac

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score 0 · Answer 6 · answered May 08 '20 at 13:12

With defined you'll have to do something like that:

if (defined("CONST_NAME"))
    $value = CONST_NAME;

This will work, but you'll could get an annoying error message in your code editor (in my case Visual Studio Code with PHP Inteliphense extension) for the second line, since it wont find CONST_NAME. Another alternative would be to use the constant function. It takes an string as the constant name and returns null if the constant is not defined:

$value = constant("CONST_NAME");
if ($value != null)
{
    // Use the value ...
}

Since you passed the const name as a string, it wont generate an error on the code editor.

This will rise a warning if it is already defined. – H Aßdøµ Apr 29 '21 at 11:42 — H Aßdøµ, Apr 29 '21 at 11:42

How to check if a defined constant exists in PHP?

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