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Constants

Constants

This lesson discusses all there is to know about constants, i.e., defining them, checking if they're defined and using them in PHP programs.

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Defining Constants #

Constants, by definition, are variables that cannot be modified during the execution of the script. They are created using the const statement or the define function.

Convention: In PHP, the convention is to use uppercase letters for constant names, e.g., A_CONSTANT.

The following code snippet shows how constants are created:

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<?php
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 already defined one constant, you can define another one based on it:

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<?php
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
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"],BETTER_APP_LANGUAGES));
?>

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