PHP - 在类中定义常量

Question

如何在类中定义常量，并使其仅在类上下文中调用时可见？

....类似于 Foo::app()->MYCONSTANT;

(如果像 MYCONSTANT 那样调用会被忽略)

Answer 1

见 Class Constants :

class MyClass
{
    const MYCONSTANT = 'constant value';

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

echo MyClass::MYCONSTANT. "\n";

$classname = "MyClass";
echo $classname::MYCONSTANT. "\n"; // As of PHP 5.3.0

$class = new MyClass();
$class->showConstant();

echo $class::MYCONSTANT."\n"; // As of PHP 5.3.0

在这种情况下，单独回显 MYCONSTANT 会引发关于未定义常量的通知，并输出转换为字符串的 constant 名称:"MYCONSTANT".

---

编辑 - 也许您正在寻找的是 static properties / variables :

class MyClass
{
    private static $staticVariable = null;

    public static function showStaticVariable($value = null)
    {
        if ((is_null(self::$staticVariable) === true) && (isset($value) === true))
        {
            self::$staticVariable = $value;
        }

        return self::$staticVariable;
    }
}

MyClass::showStaticVariable(); // null
MyClass::showStaticVariable('constant value'); // "constant value"
MyClass::showStaticVariable('other constant value?'); // "constant value"
MyClass::showStaticVariable(); // "constant value"