scope resolution operator

基本解释

You can access a static property using the scope resolution operator, which consists of a double colon (::).
可以使用範圍解析操作符來訪問靜態屬性，該操作符由雙冒號 (::) 組成。
Notice that both methods refer to the $iodir property using the keywordself and the scope resolution operator.
注意，兩個方法都使用關鍵字 self 和訪問解析操作符來引用 $iodir 屬性。
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.
要從類外部調用靜態方法，可使用類名加上範圍解析符和方法名。