用于子类的PHP依赖项注入

我是php oop的新手，我正在努力以正确的方式学习它，所以我对下面的代码有两个问题：

1. 当我创建对象Circle和Square时，我正在通过Circle和Square的构造函数传递Database对象。通过这样做，我是在内存中创建和使用一个数据库对象还是两个数据库对象？我之所以问这个问题，是因为当我打印包含这些对象的数组时，我得到了这两个对象的答案：

   [连接：数据库：私有]=>资源id#4

2. 从依赖注入的角度来看，这对于这个特定的案例来说是一个好的实践吗？

代码：

<?php
class Database{
    public $server = "localhost";
    private $user = "root";
    private $pass = "mysql";
    private $db = "ao";
    private $connection;
    function __construct(){
        $this->openConnection();
    }
    public function openConnection(){
        $this->connection = mysql_connect($this->server, $this->user, $this->pass);
        if(!$this->connection){
            die("Could not connect: " . mysql_error());
        }
        else{
            $db = mysql_select_db($this->db, $this->connection);
            if(!$db){
                die("Could not connect: " . mysql_error());
            }
        }
    }
    public function closeConnection(){
        if (isset($this->connection)){
            mysql_close($this->connection);
            unset($this->connection);
        }
    }
    public function query($sql){
        $result = mysql_query($sql, $this->connection);
        $this->confirmQuery($result);
        return $result;
    }
    public function fetchAssoc($result){
        return mysql_fetch_assoc($result);
    }
    public function numRows($result){
        return mysql_num_rows($result);
    }
    private function confirmQuery($result){
        if(!$result){
            die("Query failed: " . mysql_error());
        }
    }
}
$database = new Database;
?>
<?php
    include("db.class.php");
//////////////////////////////////////////////////////
    abstract class GeoShape{
        private $db;
        abstract public function area();
    }
//////////////////////////////////////////////////////
    class Circle extends GeoShape{
        private $r = 5;

        public function __construct(Database $db){
            $this->db = $db;
        }
        public function area(){
            return $this->r * $this->r * 3.14;
        }
    ::
//////////////////////////////////////////////////////////
    class Square extends GeoShape{
        private $s = 5;

        public function __construct(Database $db){
            $this->db = $db;
        }
        public function area(){
            return $this->s * $this->s;
        }
    }
    $objects = array(new Circle($database), new Square($database));
    foreach ($objects as $object){
        echo "Area of the " . get_class($object) . ": " . $object->area() . "<br>";
    }
///////////////////////////////////////////////////////////////
    echo "<pre>";
    print_r($objects);
?>