My preferred way of checking if a constant is set, and if it isn't - setting it (could be used to set defaults in a file, where the user has already had the opportunity to set their own values in another.)

<?php

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

?>

Dan.

// Checking the existence of a class constant, if the class is referenced by a variable.

class Class_A
{
    const CONST_A = 'value A';
}

// When class name is known.
if ( defined( 'Class_A::CONST_A' ) )
    echo 'Class_A::CONST_A defined';

// Using a class name variable. Note the double quotes.
$class_name = Class_A::class;
if ( defined( "$class_name::CONST_A" ) )
    echo '$class_name::CONST_A defined';

// Using an instantiated object for a variable class.
$object_A = new $class_name();
if ( defined( get_class($object_A).'::CONST_A' ) )
    echo '$object_A::CONST_A defined';

if you want to check id a class constant is defined use self:: before the constant name:

<?php
defined('self::CONSTANT_NAME');
?>

Before using defined() have a look at the following benchmarks:

true                                       0.65ms
$true                                      0.69ms (1)
$config['true']                            0.87ms
TRUE_CONST                                 1.28ms (2)
true                                       0.65ms
defined('TRUE_CONST')                      2.06ms (3)
defined('UNDEF_CONST')                    12.34ms (4)
isset($config['def_key'])                  0.91ms (5)
isset($config['undef_key'])                0.79ms
isset($empty_hash[$good_key])              0.78ms
isset($small_hash[$good_key])              0.86ms
isset($big_hash[$good_key])                0.89ms
isset($small_hash[$bad_key])               0.78ms
isset($big_hash[$bad_key])                 0.80ms

PHP Version 5.2.6, Apache 2.0, Windows XP

Each statement was executed 1000 times and while a 12ms overhead on 1000 calls isn't going to have the end users tearing their hair out, it does throw up some interesting results when comparing to if(true):

1) if($true) was virtually identical
2) if(TRUE_CONST) was almost twice as slow - I guess that the substitution isn't done at compile time (I had to double check this one!)
3) defined() is 3 times slower if the constant exists
4) defined() is 19 TIMES SLOWER if the constant doesn't exist!
5) isset() is remarkably efficient regardless of what you throw at it (great news for anyone implementing array driven event systems - me!)

May want to avoid if(defined('DEBUG'))...

You can use the late static command "static::" withing defined as well. This example outputs - as expected - "int (2)"

<?php
  abstract class class1
  {
    public function getConst()
    {
      return defined('static::SOME_CONST') ? static::SOME_CONST : false;
    }
  }
  
  final class class2 extends class1
  {
    const SOME_CONST = 2;
  }
  
  $class2 = new class2;
  
  var_dump($class2->getConst());
?>

I saw that PHP doesn't have an enum function so I created my own. It's not necessary, but can come in handy from time to time.

<?php
    function enum()
    {
        $args = func_get_args();
        foreach($args as $key=>$arg)
        {
            if(defined($arg))
            {
                 die('Redefinition of defined constant ' . $arg);
            }

            define($arg, $key);
        }
    }
    
    enum('ONE','TWO','THREE');
    echo ONE, ' ', TWO, ' ', THREE;
?>

This function, along with constant(), is namespace sensitive. And it might help if you imagine them always running under the "root namespace":

<?php
namespace FOO\BAR
{
    const WMP="wmp";
    function test()
    {
        if(defined("WMP")) echo "direct: ".constant("WMP"); //doesn't work;
        elseif(defined("FOO\\BAR\\WMP")) echo "namespace: ".constant("FOO\\BAR\\WMP"); //works
        echo WMP; //works
    }
}
namespace
{
    \FOO\BAR\test();
}

Dont forget to put the name of your constant into single quotation mark. You will not get an error or a warning.

<?php
define("AMOUNT_OF_APPLES", 12);
if(defined(AMOUNT_OF_APPLES)){
   //you won't get an output here
   echo AMOUNT_OF_APPLES;
}
?>

so do instead

<?php
define("AMOUNT_OF_APPLES", 12);
if(defined("AMOUNT_OF_APPLES")){
   //here you go
   echo AMOUNT_OF_APPLES;
}

//output: 12
?>

It took me half an day to see it...

I found something out: defined() becomes probably false if a reference gets lost.

<?php

session_start(); // $_SESSION created
define('SESSION_BACKUP', $_SESSION);
if (defined('SESSION_BACKUP')) echo 'A';
session_unset(); // $_SESSION destroyed
if (defined('SESSION_BACKUP')) echo 'B';

?>

You will see "A", but not "B".

In PHP5, you can actually use defined() to see if an object constant has been defined, like so:

<?php

class Generic
{
    const WhatAmI = 'Generic';
}

if (defined('Generic::WhatAmI'))
{
    echo Generic::WhatAmI;
}

?>

Thought it may be useful to note.

-Nick

If a constant's name has a leading backslash (\), it's not possible to detect its existence using the defined() function, or to get its value using the constant() function. 

You can check its existence and get its value using the get_defined_constants() function, or prepend 2 more backslashes (\\) to the constant's name.

<?php
    define('\DOMAIN', 'wuxiancheng.cn');
    $isDefined = defined('\DOMAIN'); // false
    $domain = constant('\DOMAIN'); // NULL, in Php 8+ you'll get a Fatal error.
    var_dump($isDefined, $domain);
?>

<?php
    define('\DOMAIN', 'wuxiancheng.cn');
    $constants = get_defined_constants();
    $isDefined = isSet($constants['\DOMAIN']);
    $domain = $isDefined ? $constants['\DOMAIN'] : NULL;
    var_dump($isDefined, $domain);
?>

<?php
    define('\DOMAIN', 'wuxiancheng.cn');
    $isDefined = defined('\\\DOMAIN');
    $domain = constant('\\\DOMAIN');
    var_dump($isDefined, $domain);
?>

If you wish to protect files from direct access I normally use this:

index.php:

<?php
// Main stuff here
define('START',microtime());

include "x.php";
?>

x.php:

<?php
defined('START')||(header("HTTP/1.1 403 Forbidden")&die('403.14 - Directory listing denied.'));
?>

Be careful with boolean defines and assuming a check is done for a specific value by defined such as
<?php

define('DEBUG', false);

if(defined('DEBUG')){
    echo 'Not really debugging mode';
}
?>

You want to also check the constant as in

<?php
define('DEBUG', true);

if(defined('DEBUG') && DEBUG){
    echo 'Really this is debugging mode';
}
?>

All defined is doing is verifying the constant exists not it's value.

To check in a trait if a constant is defined in the class using the trait, prepend the constant name with `self::class`:

<?php

trait MyTrait {
  public function checkConstant() {
    assert(defined(self::class . "::MY_CONSTANT"));
    print self::MY_CONSTANT;
  }
}

class MyClass {
  use MyTrait;
  protected const MY_CONSTANT = 'my value';
}

$class = new MyClass();
$class->checkConstant();
?>

You may find that if you use <?= ?> to dump your constants, and they are not defined, depending on your error reporting level, you may not display an error and, instead, just show the name of the constant. For example:

<?= TEST ?>

...may say TEST instead of an empty string like you might expect. The fix is a function like this:

<?php

function C(&$constant) {
    $nPrev1 = error_reporting(E_ALL);
    $sPrev2 = ini_set('display_errors', '0');
    $sTest = defined($constant) ? 'defined' : 'not defined';
    $oTest = (object) error_get_last();
    error_reporting($nPrev1);
    ini_set('display_errors', $sPrev2);
    if ($oTest->message) {
        return '';
    } else {
        return $constant;
    }
}

?>

And so now you can do:

<?= C(TEST) ?>

If TEST was assigned with define(), then you'll receive the value. If not, then you'll receive an empty string.

Please post if you can do this in fewer lines of code or do something more optimal than toggling the error handler.