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 W3Schools.com!");
echo GREETING;Example
This example declares a constant inside a function, and output it outside the function:
function myTest() {
define("GREETING", "Welcome to W3Schools.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 W3Schools.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];
