start work with PHP OOP
<?php
//Config
$db_user = 'root';
$db_pass = '';
$db_host = 'localhost';
$db_name = 'test';

class Database
{
    private $Database;
    private static $instance;

    public static function instance()
    {
        if ( !self::$instance )
            self::$instance = new Database();
        return self::$instance;
    }

    public function connect($host, $user, $password, $name)
    {
        $this->db_link = mysql_connect($host, $user, $password);
        mysql_set_charset('utf8');
        mysql_select_db($name, $this->db_link);
    }

    public function query($quory)
    {
        $sql = mysql_query($quory);
        $row = mysql_fetch_array($sql);

        return $row;
    }
}

class Book extends Database
{
    public function getData2()
    {
        $sql = $this->query('SELECT * FROM users WHERE price = "7"');
        return $sql['name'];
    }
}

$db = Database::instance();
$db->connect($db_host, $db_user, $db_pass, $db_name);

$b = new Book();
$res = $b->getData2();
print_r($res);

?>

$myDb = new dbLayer($settings);

$myBook = new book($myDb);

$a = $myBook->getAllData($someParameter);

in the constructor of book you save the reference to the dblayer...

class book {
  var $dbTier;
  __constructor($db) {
    $this->dbTier = $db;
    ...

class Book
{
    var $db;

    function __construct() {
        $this->db = Database::instance();
    }
}