<?php

class AAA{}
class BBB extends AAA{}

class X{
    protected AAA $foo;
    function __construct(AAA $foo){
        $this->foo = $foo;
    }
	function set(AAA $foo){
		$this->foo = $foo;
	}
}

class Y extends X{
    protected BBB $foo;
    function __construct(BBB $foo){
        parent::__construct($foo);
    }
	function set(BBB $foo){ // cannot do this
		$this->foo = $foo;
	}
}