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.

Its 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

<?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.