php singleton数据库连接,这个代码是不好的做法？

我正在尝试创建一个简单的使用单例类连接到mysql数据库并进行查询,代码工作正常,我没有遇到任何问题,但因为我是OOP的新手,我想知道这是否是不好的做法.

这是班级

class Database {
private $databaseName = 'dbname';
private $host = 'localhost';
private $user = 'user';
private $password = 'pass'; 
private static $instance; //store the single instance of the database

private function __construct(){
    //This will load only once regardless of how many times the class is called
    $connection = mysql_connect($this->host,$this->user,$this->password) or die (mysql_error());
    $db = mysql_select_db($this->databaseName,$connection) or die(mysql_error()); 
    echo 'DB initiated

$query = "SELECT * FROM registrations";
echo $database->numrows($query);
$sql = $database->query($query);