Domanda / i sulle connessioni PHP DOP

Diciamo che ho una classe Db [db.php]:

<?php

    class Db
    {

        // class properties
        private $db_driver = ''; // dsn credentials
        private $db_dhost = '';
        private $db_dname = '';
        private $db_uname = '';
        private $db_pword = '';

        private $db_tzone = ''; // timezone

        private $db_email = ''; // error reporting
        private $db_headr = '';
        private $db_ecode = 0;
        private $db_emess = '';

        private $bConnected = false;
        private $conn = null;

        public function __construct($my_env) {

            # d = development, t = testing, p = production, s = sandbox
            $my_letter = strtoupper($my_env);

            $this->db_driver = 'mysql';
            $this->db_dhost = '...';
            $this->db_dname = '...';
            $this->db_uname = '...';
            $this->db_pword = '...';

            $this->db_tzone = 'America/Los_Angeles';

            $this->db_email = '[email protected]';
            $this->db_headr = 'From: [email protected]';

            // set db type, host and db credentials from $_SERVER environment variables
            // $this->db_driver = filter_input(INPUT_SERVER, 'DB_' . $my_letter . '_DRIVER', FILTER_SANITIZE_URL);
            // $this->db_dhost = filter_input(INPUT_SERVER, 'DB_' . $my_letter . '_DHOST', FILTER_SANITIZE_URL);
            // $this->db_dname = filter_input(INPUT_SERVER, 'DB_' . $my_letter . '_DNAME', FILTER_SANITIZE_URL);
            // $this->db_uname = filter_input(INPUT_SERVER, 'DB_' . $my_letter . '_UNAME', FILTER_SANITIZE_URL);
            // $this->db_pword = filter_input(INPUT_SERVER, 'DB_' . $my_letter . '_PWORD', FILTER_SANITIZE_URL);            

        }

        public function dbConnect(){

            # initialize connection
            $this->conn = null;

            try {

                # Set default timezone
                date_default_timezone_set($this->db_tzone);

                # Set database options
                $db_options = array(
                    PDO::ATTR_EMULATE_PREPARES => false,
                    PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
                    PDO::ATTR_TIMEOUT => 150,
                    PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES utf8mb4',
                    1002 => 'SET NAMES utf8mb4'
                )

                # Create new connection
                $this->conn = new PDO($db_driver . ':host=' . $this->db_dhost . ';dbname=' . $this->db_dname . ';charset=utf8mb4', 
                    $this->db_uname, 
                    $this->db_pword, 
                    $db_options);

                # Charset ignored < PHP 5.3.6
                $this->conn->exec('set names utf8');

                # set connected to true
                $bConnected = true;

            } catch (PDOException $de) {

                # get error information
                $this->db_ecode = $de->getCode();
                $this->db_emess = $de->getMessage();

                $body = '[' . $this->db_ecode . '] [' . $this->db_emess . ']';

                # send error to admin
                mail($this->db_email, "Database Connection Error", $body, $this->db_headr);

                # set connection to false
                $this->conn = null;

                # set connected to false
                $bConnected = false;

            }

            returned array($this->conn, $bConnected);

        }

    }


?>

Per creare un'istanza, sto usando qualcosa di simile a:

// include db class
include_once 'db.php';

// use development db
$instance = new Db('d');

// capture result of attempt
list($conn, $bConnected) = $instance->dbConnect();

if($bConnected == true) {

    // perform crud
    $conn = ...

}

Le mie domande sono queste:

(a) c'è un modo migliore per astrarre le credenziali di connessione DSN diverse da $ _SERVER variabili

(b) posso migliorare questo codice di esempio, per renderlo più sicuro?

(c) dovrei implementare codice di pulizia come ...

$conn = null;

o ...

unset($conn);

se non si verificano eccezioni? E infine ...

(d) ho trascurato qualcosa di palesemente ovvio?