class dbconnect {
    protected $db_conn;
    public $db_user='xxxx';
    public $db_pass='xxxx';
    public $db_host='localhost';
    public $db_name='xxxx';

function connect() {
        try{
            $this->db_conn=new PDO("mysql:host=$this->db_host;dbname=$this->db_name",$this->db_user,$this->db_pass);
            return $this->db_conn;
        }
        catch (Exception $e){
            return $e->getMessage();
        }
    }
}


include_once ( 'class.dbconn.php' );

class Book{
    public $link;

    public function __construct(){
        $db_conn=new dbconnect();
        $this->link = $db_conn->connect();
        return $this->link;
    }
}

class new{
include_once 'classes/class.book.php';
$book = new Book();
}

public function disconnect() {
    $this->db_conn = null
}

public function __destruct() {
    $this->disconnect();
}

<?php
class dbconnect {
    protected $db_conn;
    public $db_user='xxxx';
    public $db_pass='xxxx';
    public $db_host='localhost';
    public $db_name='xxxx';

function connect() {
        try{
            $this->db_conn=new PDO("mysql:host=$this->db_host;dbname=$this->db_name",$this->db_user,$this->db_pass);
            return $this->db_conn;
        }
        catch (Exception $e){
            return $e->getMessage();
        }
    }

function disconnect()
    {
            try
            {
                $this->db_conn=null;
                return $this->db_conn;
            }
            catch (Exception $e){
                return $e->getMessage();
            }
            }
    }
}


include_once ( 'class.dbconn.php' );

class Book{
    public $link;

    public function __construct(){
        $db_conn=new dbconnect();
        $this->link = $db_conn->connect();
        return $this->link;
    }
}

class new{
include_once 'classes/class.book.php';
$book = new Book();

$close = new dbconnect();
// this will close connection
$close->disconnect();


}?>