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PHP Constants

Defining constants

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

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

And a Notice will be issued:

Constant ... already defined in ...

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 arraydereferencing), 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

Checking if constant is defined

Simple check
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 here 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
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',

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]; // 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

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