isset()函数 , unset()函数, empty() 函数是一个语言结构而非函数,因此它无法被变量函数调用.

isset()、empty() 只检测变量,检测任何非变量的东西都将导致解析错误.

isset():检测某个变量是否存在,若存在,返回真,否则返回假。
emtpy():检测某变量是否为空(null).
unset():删除某个变量。


如果需要检查一个常量是否存在,则使用
defined() 函数。

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