Constants

Syntax

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

Remarks

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.

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.

<?php

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:

<?php

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:

<?php

$constants = get_defined_constants();
var_dump($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):

<?php

$constants = get_defined_constants();

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

$new_constants = get_defined_constants();

$myconstants = array_diff_assoc($new_constants, $constants);
var_export($myconstants); 
   
/* 
Output:

array (
  'HELLO' => 'hello',
  'WORLD' => 'world',
) 
*/

It's sometimes useful for debugging

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.

<?php

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

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

Plain constant example

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', [
    1,
    VALUES,
]);

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

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("EARTH_IS_ROUND", !EARTH_IS_FLAT);
define("MORE_UNKNOWN", UNKNOWN);
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
define("MAX_SESSION_TIME_IN_MINUTES", MAX_SESSION_TIME / 60);

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


define("APP_BETTER_FUTURE_LANGUAGES", array_merge(["fr"], APP_BETTER_LANGUAGES));

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 array dereferencing), but are automatically namespace prefixed with the currently active namespace.

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

Using constants

To use the constant simply use its name:

if (EARTH_IS_FLAT) {
    print "Earth is flat";
}

print APP_ENV_UPPERCASE;

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);