PHP类中这两个符号用起来很眩晕。
PDO::ATTR_CLIENT_VERSION 这个双冒号是什么意思
回复讨论(解决方案)
访问PHP类中的成员变量或方法时，如果被引用的变量或者方法被声明成const（定义常量）或者static（声明静态），使用操作符::，，反之使用操作符->。
这样说应该明白吧。。。
手册中不是有这样的描述吗?
作用域分辨运算符（::）
有时，在没有声明任何实例的情况下访问类中的函数或者基类中的函数和变量很有用处。而 :: 运算符即用于此情况。
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