Php/docs/language.oop5.constants
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 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. 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 $classname::CONSTANT . "\n"; // As of PHP 5.3.0$class = new MyClass();$class->showConstant();echo $class::CONSTANT."\n"; // As of PHP 5.3.0?>
Example #2 Static data example
<?phpclass foo { // As of PHP 5.3.0 const BAR = <<<'EOT'barEOT; // As of PHP 5.3.0 const BAZ = <<<EOTbazEOT;}?>
Note:
Support for initializing constants with Heredoc and Nowdoc syntax was added in PHP 5.3.0.
The special ::class
constant is available as of PHP 5.5.0, and allows
for fully qualified class name resolution at compile time, this is useful for namespaced classes:
Example #3 Namespaced ::class example
<?phpnamespace foo { class bar { } echo bar::class; // foo\bar}?>
Example #4 Constant expression example
<?phpconst ONE = 1;class foo { // As of PHP 5.6.0 const TWO = ONE * 2; const THREE = ONE + self::TWO; const SENTENCE = 'The value of THREE is '.self::THREE;}?>
It is possible to provide a scalar expression involving numeric and string literals and/or constants in context of a class constant.
Note:
Constant expression support was added in PHP 5.6.0.
Example #5 Class constant visibility modifiers
<?phpclass Foo { // As of PHP 7.1.0 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.