scope resolution operator

基本解释

You can access a static property using the scope resolution operator, which consists of a double colon (::).
可以使用范围解析操作符来访问静态属性，该操作符由双冒号 (::) 组成。
Notice that both methods refer to the $iodir property using the keywordself and the scope resolution operator.
注意，两个方法都使用关键字 self 和访问解析操作符来引用 $iodir 属性。
To call a static method from outside of its class, use the class name together with the scope resolution operator and the name of the method.
要从类外部调用静态方法，可使用类名加上范围解析符和方法名。