PHP TutorialCompile PHP ExtensionsContributing to the PHP CoreContributing to the PHP ManualCreate PDF files in PHPInstalling a PHP environment on WindowsPHP Alternative Syntax for Control StructuresPHP APCuPHP Array iterationPHP ArraysPHP Asynchronous programmingPHP Autoloading PrimerPHP BC Math (Binary Calculator)PHP Built in serverPHP CachePHP Classes and ObjectsPHP ClosurePHP Coding ConventionsPHP Command Line Interface (CLI)PHP CommentsPHP Common ErrorsPHP Compilation of Errors and WarningsPHP Composer Dependency ManagerPHP ConstantsPHP Control StructuresPHP CookiesPHP CryptographyPHP DateTime ClassPHP DebuggingPHP Dependency InjectionPHP Design PatternsPHP Docker deploymentPHP Exception Handling and Error ReportingPHP Executing Upon an ArrayPHP File handlingPHP Filters & Filter FunctionsPHP Functional ProgrammingPHP FunctionsPHP GeneratorsPHP Headers ManipulationPHP How to break down an URLPHP How to Detect Client IP AddressPHP HTTP AuthenticationPHP Image Processing with GDPHP ImagickPHP IMAPPHP Installing on Linux/Unix EnvironmentsPHP JSONPHP LocalizationPHP LoopsPHP Machine learningPHP Magic ConstantsPHP Magic MethodsPHP Manipulating an ArrayPHP mongo-phpPHP Multi Threading ExtensionPHP MultiprocessingPHP MySQLiPHP MySQLi affected rows returns 0 when it should return a positive integerPHP NamespacesPHP Object SerializationPHP OperatorsPHP Output BufferingPHP Outputting the Value of a VariablePHP Parsing HTMLPHP Password Hashing FunctionsPHP PDOPHP PerformancePHP PHPDocPHP Processing Multiple Arrays TogetherPHP PSRPHP Reading Request DataPHP RecipesPHP ReferencesPHP ReflectionPHP Regular Expressions (regexp/PCRE)PHP Secure Remeber MePHP SecurityPHP Sending EmailPHP SerializationPHP SessionsPHP SimpleXMLPHP SOAP ClientPHP SOAP ServerPHP SocketsPHP SPL data structuresPHP SQLite3PHP StreamsPHP String formattingPHP String Parsing

PHP Constants

From WikiOD

Syntax[edit | edit source]

  • define ( string $name , mixed $value [, bool $case_insensitive = false ] )

Remarks[edit | edit source]

Constants are used to store the values that are not supposed to be changed later. They also are often used to store the configuration parameters especially those which define the environment (dev/production).

Constants have types like variables but not all types can be used to initialize a constant. Objects and resources cannot be used as values for constants at all. Arrays can be used as constants starting from PHP 5.6

Some constant names are reserved by PHP. These include true, false, null as well as many module-specific constants.

Constants are usually named using uppercase letters.

Defining constants[edit | edit source]

Constants are created using the const statement or the define function. The convention is to use UPPERCASE letters for constant names.

Define constant using explicit values[edit | edit source]

const PI = 3.14; // float
define("EARTH_IS_FLAT", false); // boolean
const "UNKNOWN" = null; // null
define("APP_ENV", "dev"); // string
const MAX_SESSION_TIME = 60 * 60; // integer, using (scalar) expressions is ok

const APP_LANGUAGES = ["de", "en"]; // arrays

define("BETTER_APP_LANGUAGES", ["lu", "de"]); // arrays

Define constant using another constant[edit | edit source]

if you have one constant you can define another one based on it:

const TAU = PI * 2;
define("APP_ENV_UPPERCASE", strtoupper(APP_ENV)); // string manipulation is ok too
// the above example (a function call) does not work with const:
// const TIME = time(); # fails with a fatal error! Not a constant scalar expression

const APP_FUTURE_LANGUAGES = [-1 => "es"] + APP_LANGUAGES; // array manipulations


Reserved constants[edit | edit source]

Some constant names are reserved by PHP and cannot be redefined. All these examples will fail:

define("true", false); // internal constant
define("false", true); // internal constant
define("CURLOPT_AUTOREFERER", "something"); // will fail if curl extension is loaded

And a Notice will be issued:

Constant ... already defined in ...

Conditional defines[edit | edit source]

If you have several files where you may define the same variable (for example, your main config then your local config) then following syntax may help avoiding conflicts:

defined("PI") || define("PI", 3.1415); // "define PI if it's not yet defined"

const vs define[edit | edit source]

define is a runtime expression while const a compile time one.

Thus define allows for dynamic values (i.e. function calls, variables etc.) and even dynamic names and conditional definition. It however is always defining relative to the root namespace.

const is static (as in allows only operations with other constants, scalars or arrays, and only a restricted set of them, the so called constant scalar expressions, i.e. arithmetic, logical and comparison operators as well as array dereferencing), but are automatically namespace prefixed with the currently active namespace.

const only supports other constants and scalars as values, and no operations.

Class Constants[edit | edit source]

Constants can be defined inside classes using a const keyword.

class Foo {
    const BAR_TYPE = "bar";

    // reference from inside the class using self::
    public function myMethod() {
        return self::BAR_TYPE;

// reference from outside the class using <ClassName>::
echo Foo::BAR_TYPE;

This is useful to store types of items.


class Logger {
    const LEVEL_INFO = 1;
    const LEVEL_WARNING = 2;
    const LEVEL_ERROR = 3;

    // we can even assign the constant as a default value
    public function log($message, $level = self::LEVEL_INFO) {
        echo "Message level " . $level . ": " . $message;

$logger = new Logger();
$logger*>log("Info"); // Using default value
$logger*>log("Warning", $logger::LEVEL_WARNING); // Using var
$logger*>log("Error", Logger::LEVEL_ERROR); // using class

Checking if constant is defined[edit | edit source]

Simple check[edit | edit source]

To check if constant is defined use the defined function. Note that this function doesn't care about constant's value, it only cares if the constant exists or not. Even if the value of the constant is null or false the function will still return true.


define("GOOD", false);

if (defined("GOOD")) {
    print "GOOD is defined" ; // prints "GOOD is defined"

    if (GOOD) {
        print "GOOD is true" ; // does not print anything, since GOOD is false

if (!defined("AWESOME")) {
   define("AWESOME", true); // awesome was not defined. Now we have defined it 

Note that constant becomes "visible" in your code only after the line where you have defined it:


if (defined("GOOD")) {
   print "GOOD is defined"; // doesn't print anyhting, GOOD is not defined yet.

define("GOOD", false);

if (defined("GOOD")) {
   print "GOOD is defined"; // prints "GOOD is defined"

Getting all defined constants[edit | edit source]

To get all defined constants including those created by PHP use the get_defined_constants function:


$constants = get_defined_constants();
var_dump($constants); // pretty large list

To get only those constants that were defined by your app call the function at the beginning and at the end of your script (normally after the bootstrap process):


$constants = get_defined_constants();

define("HELLO", "hello"); 
define("WORLD", "world"); 

$new_constants = get_defined_constants();

$myconstants = array_diff_assoc($new_constants, $constants);


array (
  'HELLO' => 'hello',
  'WORLD' => 'world',

It's sometimes useful for debugging

Constant arrays[edit | edit source]

Arrays can be used as plain constants and class constants from version PHP 5.6 onwards:

Class constant example[edit | edit source]

class Answer {
    const C = [2,4];

print Answer::C[1] . Answer::C[0]; // 42

Plain constant example[edit | edit source]

const ANSWER = [2,4];
print ANSWER[1] . ANSWER[0]; // 42

Also from version PHP 7.0 this functionality was ported to the define function for plain constants.

define('VALUES', [2, 3]);
define('MY_ARRAY', [

print MY_ARRAY[1][1]; // 3

Using constants[edit | edit source]

To use the constant simply use its name:

    print "Earth is flat";


or if you don't know the name of the constant in advance, use the constant function:

// this code is equivalent to the above code
$const1 = "EARTH_IS_FLAT";
$const2 = "APP_ENV_UPPERCASE";

if (constant($const1)) {
    print "Earth is flat";

print constant($const2);