1,第一种定义常量
![](https://images2017.cnblogs.com/blog/1065651/201712/1065651-20171223020715631-720353474.png)
![](https://images2017.cnblogs.com/blog/1065651/201712/1065651-20171223020715834-870224629.png)
![](https://images2017.cnblogs.com/blog/1065651/201712/1065651-20171223020715928-150661972.png)
2,检测常量是否存在
![](https://images2017.cnblogs.com/blog/1065651/201712/1065651-20171223020716100-1145066028.png)
3,应用
可以作为引用是否有权限引用一个文件的开关,例如:
在a.php页面中
![](https://images2017.cnblogs.com/blog/1065651/201712/1065651-20171223020716553-1573330603.png)
此处应该是 if(defined(ENTER_TG)){ exit('无权限调用!'); }
![](https://images2017.cnblogs.com/blog/1065651/201712/1065651-20171223020716709-1760012374.png)
4.第二种定义方式 const 常量名=常量值;
在b.php页面中
![](https://images2017.cnblogs.com/blog/1065651/201712/1065651-20171223020716709-1760012374.png)
4.第二种定义方式 const 常量名=常量值;
const PI=3.14;
5.第二种使用方式 constant("常量名")
product= constant("PI") * 3;