Fundamentals: Constants #

A constant is a named value that cannot be changed.

There are two types of constants:

• namespace constant - A constant declared in a namespace.
• class constant - A constant declared in a class, interface, or enum.

Namespace Constants #

To declare a namespace constant, use the const keyword.

namespace Foo;

const u8 MY_CONST = 1;

To access a namespace constant, refer to it by its fully qualified name:

$value = Foo\MY_CONST;

or import it using the use keyword:

namespace Bar;

use const Foo\MY_CONST;

$value = MY_CONST;

Class Constants #

To declare a class constant, use the const keyword within a class, interface, or enum.

class MyClass {
    const u8 MY_CONST = 1;
}

interface MyInterface {
    const u8 MY_CONST = 1;
}

enum MyEnum {
    const u8 MY_CONST = 1;
}

To access a class constant, you can use the :: operator:

$value = MyClass::MY_CONST;

Unlike namespace constants, class constants can have modifiers:

• public - The constant is accessible from anywhere.
• protected - The constant is accessible from within the class and its subclasses.
• private - The constant is accessible only from within the class.
• final - The constant cannot be overridden by subclasses.

class MyClass {
    public const u8 MY_CONST = 1;
    protected const u8 MY_CONST = 1;
    private const u8 MY_CONST = 1;
    final const u8 MY_CONST = 1;
}