没有`new`关键字实例化PHP类？

I am used to the new keyword, so I was surprised to find that the following works for instantiation as well.

class MyClass
{
    const CONSTANT = 'constant value';

    function showConstant() {
        echo  self::CONSTANT;
    }
}

// $classname = new MyClass;
$classname = "MyClass";

echo $classname::CONSTANT; 

I can't seem to find any documentation pertaining to this online. Would someone help me out?

As of PHP 5.3.0, it's possible to reference the class using a variable [0]

You're not creating an instance of an object. So you're not instantiating a class. PHP constants can be access statically.

[0] http://php.net/manual/en/language.oop5.constants.php

This is not an instantiation, with the four dots (::) you can access an static variable, method or constant in this case.

In fact $classname is just another way to say MyClass. So $classname::CONSTANT is the same as MyClass::CONSTANT. But there is not instantiation going on, as doing $classname->showConstant() would not work!!