PHP 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

<?phpclass MyClass{
    const CONSTANT = 'constant value';

    function showConstant() {
        echo  self::CONSTANT . "\n";
    }
}

echo MyClass::CONSTANT . "\n";
$classname = "MyClass";
echo $classname::CONSTANT . "\n"; // As of PHP 5.3.0
$class = new MyClass();$class->showConstant();

echo $class::CONSTANT."\n"; // As of PHP 5.3.0?>

Example #2 Static data example

<?phpclass foo {
    // As of PHP 5.3.0
    const BAR = <<<'EOT'barEOT;
    // As of PHP 5.3.0
    const BAZ = <<<EOTbazEOT;
}?>

Note:

Support for initializing constants with Heredoc and Nowdoc syntax was added in PHP 5.3.0.

Example #3 Constant expression example

<?phpconst ONE = 1;

class foo {
    // As of PHP 5.6.0
    const TWO = ONE * 2;
    const THREE = ONE + self::TWO;
    const SENTENCE = 'The value of THREE is '.self::THREE;
}?>

It is possible to provide a scalar expression involving numeric and string literals and/or constants in context of a class constant.