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::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.
Autoloading Classes →
? 1997–2017 The PHP Documentation GroupLicensed under the Creative Commons Attribution License v3.0 or later.
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