Constants are like variables, except that once they are defined they cannot be changed or undefined.

PHP Constants

A constant is an identifier (name) for a simple value. The value cannot be changed during the script.

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

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

Create a PHP Constant

To create a constant, use 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.

define("GREETING", "Welcome to W3Schools.com!");
echo GREETING;

define("GREETING", "Welcome to W3Schools.com!", true);
echo greeting;

PHP const Keyword

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

const MYCAR = "Volvo";
echo MYCAR;

PHP Constant Arrays

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

define("cars", [
  "Alfa Romeo",
  "BMW",
  "Toyota"
]);
echo cars[0];

Constants are Global

Constants are automatically global and can be used across the entire script.

define("GREETING", "Welcome to W3Schools.com!");

function myTest() {
  echo GREETING;
}

myTest();