<?php

class father
{
    public $var_a = 1;
    public function fn_a()
    {
        var_dump($this);
    }
}

class Son extends father
{
    public $var_b = 2;
    public function fn_b()
    {
        // var_dump($this);
        var_dump(array_keys(get_object_vars($this)));
    }
}

$son = new Son();
$son->fn_b();
// $son->fn_a();