PHPConstants

PHP Constants

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

PHP constants can be defined with thedefine() function or theconst keyword.

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

PHP define() Function

Thedefine() function defines a constant at run-time.

Thedefine() 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

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

function myTest() {  define("GREETING", "Welcome to W3Schools.com!");}myTest();echo GREETING;

PHP const Keyword

Theconst keyword defines a constant at compile-time.

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

Constants defined with theconst keyword are case-sensitive.

Syntax

constCONSTANT_NAME =value;

const GREETING = "Welcome to W3Schools.com!";echo GREETING;

PHP Array Constants

You can also create Array constants withdefine() orconst:

// Using define()define("CARS", array("Volvo", "BMW", "Toyota"));echo CARS[0];// Using constconst ANIMALS = array("Cat", "Dog", "Horse");echo ANIMALS[1];