Class Constants
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.
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https://www.php.net/manual/en/language.oop5.constants.php