php中的 类常量 Class Constants

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It is possible to define constant values on a per-class basis remaining the same and unchangeable.
　　Constants differ from normal variables in that you don't use the $ symbol to declare or use them. Like static members, constant values cannot be accessed from an instance of the object (using $object::constant).

说明： const 常量 不用 $ 符号 　　const 不加关键字 private 修饰 //Like static members, 　　不能通过 对象访问。 只能通过class访问 　　 　　 　　例子 19-15. Defining and using a constant <?php class MyClass { const constant = 'constant value'; function showConstant() { echoself::constant . "/n"; } } echo MyClass::constant . "/n"; $class = new MyClass(); $class->showConstant(); // echo $class::constant;is not allowed ?>