php如何在一个类中引入另外一个类

class a {
    function b($obj) {
        $obj->test();
    }
}
  
class b {
    function test() {
        echo 'test';
    }
}
  
$a = new a();
$b->b(new b());

class b {
    function __construct(){
      
    }
  
    function testb(){
        echo 'test';
    }
}
class a extends b {
    function __construct(){
        parent::testb();
        //or like this
        $this->testb();
    }
    //重复定义 将会覆盖
    function testb(){
        echo 123;
    }
}
  
$a = new a();