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. Like static members, constant values cannot be accessed from an instance of the object (using $object::constant).

The value must be a constant expression, not (for example) a variable, a class member, result of a mathematical operation or a function call.

例子 19-15. Defining and using a constant


<?php
class MyClass
{
    const constant = 'constant value';

    function showConstant() {
        echo  self::constant . "\n";
    }
}

echo MyClass::constant . "\n";

$class = new MyClass();
$class->showConstant();
// echo $class::constant;  is not allowed
?>