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PHP Constants

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

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

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

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

Conditional defines

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

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

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

Using constants

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);

Constant arrays

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

Class constant example

class Answer {
 const C = [2,4];
print Answer::C[1] . Answer::C[0];



Plain constant example

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



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];




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