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;

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;

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];

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();
 

PHP Predefined Constants

PHP has nine predefined constants that change value depending on where they are used, and therefor they are called "magic constants".

These magic constants are written with a double underscore at the start and the end, except for the ClassName::class constant.

Magic Constants

Here are the magic constants, with descriptions and examples:

Constant Description
__CLASS__ If used inside a class, the class name is returned.
__DIR__ The directory of the file.
__FILE__ The file name including the full path.
__FUNCTION__ If inside a function, the function name is returned.
__LINE__ The current line number.
__METHOD__ If used inside a function that belongs to a class, both class and function name is returned.
__NAMESPACE__ If used inside a namespace, the name of the namespace is returned.
__TRAIT__ If used inside a trait, the trait name is returned.
ClassName::class Returns the name of the specified class and the name of the namespace, if any.

Note:

These constants are case-insensitive, meaning __LINE__ returns the same as __line__.