<?php
class 🔥fire {
    private $temperature = 5000;
    public function __serialize(): array       { return ['t' => $this->temperature]; }
    public function __unserialize(array $data) { $this->temperature = $data['t'];    }
}

class ⭐star extends 🔥fire {
    private $size = 695510000;
    public function __serialize(): array {
        return [
            's' => $this->size,
            '🔥fire' => parent::__serialize(),
        ];
    }
    public function __unserialize(array $data) {
        parent::__unserialize( $data['🔥fire'] );
        $this->size = $data['s'];
    }
}
echo serialize( $test = new ⭐star );