在 PHP 语言中，我们可以通过类型运算符 instanceof 判断某个变量是否属于指定类或接口的实例

<?php
interface I {
     public  function hello();
}

class A implements  I {
     public function hello()
     {
         // TODO: Implement hello() method.
          print ("Hello from A\n");
     }
}

class B extends  A {
     public function  greeter (){
         print("Hello from B\n");
     }
}
$a=new A;
$b=new B;
var_dump($b  instanceof B);
var_dump($b instanceof A);
var_dump($b instanceof I);