PHP Constants

PHP Constants

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

PHP constants can be defined with the define() function or the const keyword.

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

PHP define() Function

The define() function defines a constant at run-time.

The define() function always define constants in the global scope, and can be declared inside conditional blocks (if/else, loops, functions), but they cannot be used to define class constants.

Syntax

define(CONSTANT_NAME, value);

Parameters:

• CONSTANT_NAME - Specifies the name of the constant (the name is case-sensitive)
• value - Specifies the value of the constant

Example

Create a constant named "GREETING" with define():

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

Example

This example declares a constant inside a function, and output it outside the function:

function myTest() {
  define("GREETING", "Welcome to example.com!");
}

myTest();

echo GREETING;

PHP const Keyword

The const keyword defines a constant at compile-time.

Constants defined with the const keyword must be declared at the top-level scope because they are defined at compile-time. This means that they cannot be declared inside functions, loops, if/else statements or try/catch blocks.

Constants defined with the const keyword can be used to define class constants.

Constants defined with the const keyword are case-sensitive.

Syntax

const CONSTANT_NAME = value;

Example

Create a constant named "GREETING" with const:

const GREETING = "Welcome to example.com!";
echo GREETING;

PHP Array Constants

You can also create Array constants with define() or const:

Example

Create Array constants:

// Using define()
define("CARS", array("Volvo", "BMW", "Toyota"));
echo CARS[0];

// Using const
const ANIMALS = array("Cat", "Dog", "Horse");
echo ANIMALS[1];