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.
Defining and using a constant
showConstant();
echo $class::CONSTANT."\n"; // As of PHP 5.3.0
?>
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Static data example
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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:
Namespaced ::class example
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Constant expression example
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It is possible to provide a scalar expression involving numeric and string literals and/or constants in context of a class constant.
Constant expression support was added in PHP 5.6.0.
Class constant visibility modifiers
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&example.outputs.71;
As of PHP 7.1.0 visibility modifiers are allowed for class constants.