How to call a function of an outside class in a class instance that belongs to another class instance

The title may not be described clearly below is the pseudo-code

Class A {public    $xx = 0;    public $b _list = [];    Public Function Add_b ($param) {        $this->b_list [] = new B ($param);    } Public Function pp () {$xx ++}}class B extend a {public    $set = 0;    Public Function __init ($param) {        //Forget the constructor how to write this is believed to be a constructor        $set = $param;    }    Public Function pp () {        //want to make Class A $xx +        //...           }} $a = new A; $a->add_b (111); $a->add_b (222);//now $a has 2 $b//The following is the invocation of the first $b$a->b_list[0]->pp ();//How to implement $a in pp- >xx + +  ?

Reply to discussion (solution)

Public Function pp () {
$ this->xx++
}

Forget the __construct of the constructor function, you can use the class name directly
Class B extend a {
Public $set = 0;
Public Function B ($param) {

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