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

<?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::BARPHP_EOL;
echo 
Foo::BAZPHP_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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