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
<?phpclass 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
<?phpnamespace foo { class bar { } echo bar::class; // foo\bar}?>
Example #3 Class constant expression example
<?phpconst 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
<?phpclass 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.