class DB {

    protected $pdo;
    protected $engine    = 'mysql';
    protected $host      = 'localhost';
    protected $db_name   = 'test';
    protected $db_user   = 'root';
    protected $db_passwd = '';

    public function connect() {
        try {
            $this->pdo = new PDO($this->engine.':host='.$this->host.';dbname='.$this->db_name,$this->db_user,$this->db_passwd);
            $this->pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
            return $this->pdo;
        }   catch(PDOException $e) {
            die($e->getMessage());
        }
    }
}

class user {

    public $link;

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

}

$obj = new user();
$obj->SelectAll();

// and so on..

public function register($username, $email, $passwd, $re_passwd) {
    # fix email and passwd regex
    $reg_error = array();
    $regex_username = '/[a-zA-Z0-9-._]{6,20}$/';
    $regex_email = '/[a-zA-Z0-9._-]{3,}@[a-zA-Z0-9]{3,}.[a-zA-Z.]{2,}$/';
    if (empty($username) || empty($email) || empty($passwd) || empty($re_passwd)) {
        $reg_error['fields'] = 'All fields are required';
    }   else {
        if (!preg_match($regex_username, $username)) {
                $reg_error['username'] = 'Username must be at least 6 characters';
        }   if (!preg_match($regex_email, $email)) {
                $reg_error['email'] = 'Invalid email';
        }   if(strlen($passwd) <= 6) {
                $reg_error['passwd'] = 'Password must be at least 6 characters long and must contain at least 1 uppercase, 1 lowercase and 1 digit value';
        }   if ($re_passwd !== $passwd) {
                $reg_error['match'] = 'Passwords do not match';
        }   if($this->usernameExists($username)) {
                $reg_error['username_exists'] = 'Username: ' .$username. ' already exists';
        }   if($this->emailExists($email)) {
                $reg_error['email_exists'] = 'Email: ' .$email. ' already exists in our database';
        }
    }
    if (!empty($reg_error)) {
        foreach($reg_error as $error => $er) {
            echo 'Error: '.$er.'<br />';
        }
    }   else {
        try {

            $query = $this->link->prepare("INSERT INTO users(username, email, password, ip_address, activation_code)
                                            VALUES(:username, :email, :passwd, :ip_addr, :code)");
            $passwd = hash('sha512', $passwd);
            $ip_addr = $_SERVER['REMOTE_ADDR'];
            $code = substr( hash('sha512', md5(rand(999, 9999))) , 15, 40);
            $params = array(':username' => $username,
                            ':email'    => $email,
                            ':passwd'   => $passwd,
                            ':ip_addr'  => $ip_addr,
                            ':code'     => $code);
            $query->execute($params);
            if ($query->rowCount() != 0) {
                redirect('index.php?lastid='.$this->lastId());
            }   else {
                    return 'There's been a tehnical problem during the registration process, please try again later.';
            }
        }   catch (PDOException $e) {
                die($e->getMessage());
        }
    }
}

engine   = "mysql"
host     = "localhost"
name     = "test"
user     = "root"
password = ""

class DB {
    private $pdo;
    private $engine;
    private $host;
    private $db_name;
    private $db_user;
    private $db_passwd;

    public function _construct($path) {
        if(!is_file($path)) {
            throw new InvalidArgumentException("Hey, that's not a file!");
        }

        $credentials = @parse_ini_file($path);

        if(empty($credentials)) {
            throw new InvalidArgumentException("Hey, that's a file, but it's not a valid INI file!");
        }

        $this->engine    = $credentials["engine"];
        $this->host      = $credentials["host"];
        $this->db_name   = $credentials["name"];
        $this->db_user   = $credentials["user"];
        $this->db_passwd = $credentials["password"];

        try {
            $this->pdo = new PDO($this->engine.':host='.$this->host.';dbname='.$this->db_name,$this->db_user,$this->db_passwd);
            $this->pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
            return $this->pdo;
        }   catch(PDOException $e) {
            throw new Exception("Boo, wrong credentials!");
        }
    }

}

$db = new DB("database.ini");

$this->engine    = $credentials["engine"];
$this->host      = $credentials["host"];
$this->db_name   = $credentials["name"];
$this->db_user   = $credentials["user"];
$this->db_passwd = $credentials["password"];

} catch(PDOException $e) {
    die($e->getMessage());
}

}   catch(PDOException $e) {
    throw new Exception("Boo, wrong credentials!");
}

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

$query = $this->link->prepare("INSERT INTO users(username, email, password, ip_address, activation_code)
                                VALUES(:username, :email, :passwd, :ip_addr, :code)");

class DB {

    ...

    public function getPDO() {
        return $this->pdo;
    }
}

public function __construct() {
    $database = new DB("database.ini");

    $this->link = $database->getPDO();
}

$database = new DB("database.ini");

public function __construct(DB $database) {
    $this->link = $database->getPDO();
}

$database = new DB("database.ini");
$user = new User($database);

$database1 = new DB("database1.ini");
$user1 = new User($database1);

$database2 = new DB("database2.ini");
$user2 = new User($database2);

class Table {
    protected $database;

    public function __construct(DB $database) {
        $this->database = $database->getPDO();
    }
}

class User extends Table {

    // only user specific functions here

}

class Article extends Table {

    // only article specific functions here

}

class Comment extends Table {

    // only comment specific functions here

}

class User {

    public function register($username, $email, $passwd, $re_passwd) {
        $link = $this->database->getPDO();

        ...

        $query = $link->prepare( ... );

        ...
    }

}