Class Constants

Properties
November 14, 2018
Autoloading Classes
November 14, 2018

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 default visibility of class constants is public.

The value must be a constant expression, not (for example) a variable, a property, 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. selfparent and static).

Note that class constants are allocated once per class, and not for each class instance.

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;
// As of PHP 5.3.0
const BAZ = <<<EOT
baz
EOT;
}
?>
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