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Constant

A valid constant name starts with a letter or underscore (no $ sign before the constant name).

Unlike variables, constants are automatically global across the entire script.

Create a PHP Constant

There are 2 ways to create a constant: 1. Using the define() function 2. Using the const keyword

const vs. define()

  • const are always case-sensitive
  • define() has has a case-insensitive option.
  • const cannot be created inside another block scope, like inside a function or inside an if statement.
  • define can be created inside another block scope.

Using the define() function.

Syntax define(name, value, case-insensitive)

Parameters:

name: Specifies the name of the constant value: Specifies the value of the constant case-insensitive: Specifies whether the constant name should be case-insensitive. Default is false. Note: Defining case-insensitive constants was deprecated in PHP 7.3. PHP 8.0 accepts only false, the value true will produce a warning.

<?php
define("GREETING", "Hello World!"); // Create a constant with a case-sensitive name
define("GREETING", "Welcome to W3Schools.com!", true);// Create a constant with a case-insensitive name
echo GREETING;
?>

PHP Constant Arrays

In PHP7, you can create an Array constant using the define() function.

<?php
define("cars", [
  "Alfa Romeo",
  "BMW",
  "Toyota"
]);
echo cars[0];
?>

PHP const Keyword

You can also create a constant by using the const keyword.

<?php
const MYCAR = "Volvo";
echo MYCAR;
?>