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, a result of a mathematical operation, 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).
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;
}
?>
Unlike heredocs, nowdocs can be used in any static data context.
Note:
Nowdoc support was added in PHP 5.3.0.