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PHP Constants


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);

Parameters:

  • name: Specifies the name of the constant
  • value: Specifies the value of the constant

Example

Create a constant with a case-sensitive name:

define("GREETING", "Welcome to W3Schools.com!");
echo GREETING;
Try it Yourself »


PHP const Keyword

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

Example

Create a case-sensitive constant with the const keyword:

const MYCAR = "Volvo";
echo MYCAR;
Try it Yourself »

const vs. define()

  • 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.

PHP Constant Arrays

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

Example

Create an Array constant:

define("cars", [
  "Alfa Romeo",
  "BMW",
  "Toyota"
]);
echo cars[0];
Try it Yourself »

Constants are Global

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

Example

This example uses a constant inside a function, even if it is defined outside the function:

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

function myTest() {
  echo GREETING;
}

myTest();
Try it Yourself »


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