[8.1.常量的含义](8.1.%E5%B8%B8%E9%87%8F%E7%9A%84%E5%90%AB%E4%B9%89.md) [8.2.常量的两种定义形式](8.2.%E5%B8%B8%E9%87%8F%E7%9A%84%E4%B8%A4%E7%A7%8D%E5%AE%9A%E4%B9%89%E5%BD%A2%E5%BC%8F.md) [8.3.常量的两种取值形式](8.3.%E5%B8%B8%E9%87%8F%E7%9A%84%E4%B8%A4%E7%A7%8D%E5%8F%96%E5%80%BC%E5%BD%A2%E5%BC%8F.md) [8.4.变量与常量的区别](8.4.%E5%8F%98%E9%87%8F%E4%B8%8E%E5%B8%B8%E9%87%8F%E7%9A%84%E5%8C%BA%E5%88%AB.md) [8.5.判断一个常量是否存在：defined();](8.5.%E5%88%A4%E6%96%AD%E4%B8%80%E4%B8%AA%E5%B8%B8%E9%87%8F%E6%98%AF%E5%90%A6%E5%AD%98%E5%9C%A8%EF%BC%9Adefined;.md) [8.6.预定义常量](8.6.%E9%A2%84%E5%AE%9A%E4%B9%89%E5%B8%B8%E9%87%8F.md) [8.7.几个魔术常量](8.7.%E5%87%A0%E4%B8%AA%E9%AD%94%E6%9C%AF%E5%B8%B8%E9%87%8F.md)