声明为static的属性无法使用实例化的类对象访问？

Question

According to the php documentation http://php.net/manual/en/language.oop5.constants.php

A property declared as static can not be accessed with an instantiated class object

But doesn't the following example show that you can access a static property from the object $foo?

class Foo{
    static $my_static = 'foo';
    function staticValue(){
        return self::$my_static;
    }        
}

$foo = new Foo();
echo $foo::$my_static;

Answer 1

The trick here is that you are using the scope resolution operator :: which always references the class. It doesn't matter if you use $foo or $this or self.

Self is a keyword in PHP that references the current level, but :: can be used on an object to gain reference to the class for that object. self:: is in the same family of keywords as parent::, think of self:: as wanting access to something at the current level of an object, and parent:: as wanting access to something below the current object.

Therefore, to access something that is defined in the class. PHP created the :: operator. If you just need something from the class that is static, then you can use self inside the class or an object reference outside it.