Class Constants

It is possible to define constants on a per-class basis remaining the same and unchangeable. The default visibility of class constants is public.

Note:

Class constants can be redefined by a child class. As of PHP 8.1.0, class constants cannot be redefined by a child class if it is defined as final.

It's also possible for interfaces to have constants. Look at the interface documentation for examples.

It's possible to reference the class using a variable. The variable's value can not be a keyword (e.g. self, parent and static).

Note that class constants are allocated once per class, and not for each class instance.

Example #1 Defining and using a constant

<?phpclass MyClass{    const CONSTANT = 'constant value';    function showConstant() {        echo  self::CONSTANT . "\n";    }}echo MyClass::CONSTANT . "\n";$classname = "MyClass";echo $classname::CONSTANT . "\n";$class = new MyClass();$class->showConstant();echo $class::CONSTANT."\n";?>

The special ::class constant allows for fully qualified class name resolution at compile time, this is useful for namespaced classes:

Example #2 Namespaced ::class example

<?phpnamespace foo {    class bar {    }    echo bar::class; // foo\bar}?>

Example #3 Class constant expression example

<?phpconst ONE = 1;class foo {    const TWO = ONE * 2;    const THREE = ONE + self::TWO;    const SENTENCE = 'The value of THREE is '.self::THREE;}?>

Example #4 Class constant visibility modifiers, as of PHP 7.1.0

<?phpclass Foo {    public const BAR = 'bar';    private const BAZ = 'baz';}echo Foo::BAR, PHP_EOL;echo Foo::BAZ, PHP_EOL;?>

Output of the above example in PHP 7.1:

bar

Fatal error: Uncaught Error: Cannot access private const Foo::BAZ in …

Note:

As of PHP 7.1.0 visibility modifiers are allowed for class constants.