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 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).
<?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 ?>
<?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.
<?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.
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