It is possible to define constants
on a per-class basis remaining the same and unchangeable.
The default visibility of class constants is public.
Note:
Class constants can be redefined by a child class. As of PHP 8.1.0, class constants cannot be redefined by a child class if it is defined as final.
It's also possible for interfaces to have constants. Look
at the interface documentation
for examples.
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).
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";
$class = new MyClass();
$class->showConstant();
echo $class::CONSTANT."\n";
?>
The special ::class constant allows
for fully qualified class name resolution at compile time,
this is useful for namespaced classes:
Example #2 Namespaced ::class example
<?php
namespace foo {
class bar {
}
echo bar::class; // foo\bar
}
?>
Example #3 Class constant expression example
<?php
const ONE = 1;
class foo {
const TWO = ONE * 2;
const THREE = ONE + self::TWO;
const SENTENCE = 'The value of THREE is '.self::THREE;
}
?>
Example #4 Class constant visibility modifiers, as of PHP 7.1.0
<?php
class Foo {
public const BAR = 'bar';
private const BAZ = 'baz';
}
echo Foo::BAR, PHP_EOL;
echo Foo::BAZ, PHP_EOL;
?>
Output of the above example in PHP 7.1:
bar Fatal error: Uncaught Error: Cannot access private const Foo::BAZ in …
Note:
As of PHP 7.1.0 visibility modifiers are allowed for class constants.