PHP: Class Constants

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 class member, result of a mathematical operation or a function call.

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


<?php

class 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


<?php

class foo {

    // As of PHP 5.3.0

    const bar = <<<'EOT'

bar

EOT;

}

?>

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

Note: Nowdoc support was added in PHP 5.3.0.