Class Constants

It is possible to define constant values on a per-class basis remaining the same and unchangeable. Constants differ from normal variables in that you don't use the $ symbol to declare or use them.

The value must be a constant expression, not (for example) a variable, a property, a result of a mathematical operation, or a function call.

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

As of PHP 5.3.0, 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).

Example #1 Defining and using a constant

class MyClass
CONSTANT 'constant value';

showConstant() {
self::CONSTANT "\n";

MyClass::CONSTANT "\n";

$classname "MyClass";
$classname::CONSTANT "\n"// As of PHP 5.3.0

$class = new MyClass();

$class::CONSTANT."\n"// As of PHP 5.3.0

Example #2 Static data example

class foo {
// As of PHP 5.3.0
const BAR = <<<'EOT'

Unlike heredocs, nowdocs can be used in any static data context.


Nowdoc support was added in PHP 5.3.0.