Понять суть ООП

<?php
abstract class db {
    private $host;
    private $name;
    private $pass;
    private $db;
 
    private function __construct() { 
    //Connect to db
    }
    
    public function query() {
    //query execute
    }
    
    public function fetch() {
    //fetche the data to object
    }
    
    public function __destruct() {
    //close connection with db
    }
 
 
}
 
 
class work_with_db extends db {
    public $host = "localhost";
    public $name = "root";
    public $pass = "";
    public $db = "mywebsite";
    
    public $db_con;
    public $result;
 
    
    
    public function __construct() { 
        $this->db_con = mysqli_connect($this->host, $this->name, $this->pass, $this->db);
        if(!$this->db_con) echo "Can't connect to DB";
    }
    
    
    public function query($q) {
        $this->resut = mysqli_query($this->db_con, $q);
        return $this->resut;
    }
    
    public function fetch($data) {
        while($row = mysqli_fetch_object($data)) {
            $d[] = (array)$row;
        }
        
        return $d;
    }
    
    public function __destruct() {
        mysqli_close($this->db_con);
    }
}