php中的this self parent
使用类里面静态（一般用关键字static）的成员，必须使用self来调用。使用self来调用静态变量必须使用:: (域运算符号)。
static关键字描述一个成员是静态的，只能用类名和self::访问，值可以更改，定义时候需要加$符号，可以用private等修饰词。const用于类成员常量定义，只能用类名和self::访问，值不能更改，定义时候变量名一般要大写，不能添加private等修饰词。
private static $count = 0; //定义一个静态属性const VERSION = 2.0; //定义一个常量
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