block8.php

class A {
    private $prop_a;
    public function __serialize(): array {
        return ["prop_a" => $this->prop_a];
    }
    public function __unserialize(array $data) {
        $this->prop_a = $data["prop_a"];
    }
}
class B extends A {
    private $prop_b;
    public function __serialize(): array {
        return [
            "prop_b" => $this->prop_b,
            "parent_data" => parent::__serialize(),
        ];
    }
    public function __unserialize(array $data) {
        parent::__unserialize($data["parent_data"]);
        $this->prop_b = $data["prop_b"];
    }
}