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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

 <?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.