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
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));
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 ...
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"
define is a runtime expression while
const a compile time one.
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.