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
<?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;
}
?>
Note:
Support for initializing constants with Heredoc and Nowdoc syntax was added in PHP 5.3.0.
The special ::class constant are available as of PHP 5.5.0, and allows
for fully qualified class name resolution at compile, this is useful for namespaced classes:
Example #3 Namespaced ::class example
<?php
namespace foo {
class bar {
}
echo bar::class; // foo\bar
}
?>
Example #4 Constant expression example
<?php
const 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
<?php
class 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.