Метод за MySQLi prepared statements

<?php
namespace CCF\DB;
class MySQLi{
	private static $instance = Null;
	private $results;
	private $db;
	private $DB_HOST = 'localhost';
	private $DB_USER = 'root';
	private $DB_PASS = '';
	private $DB_NAME = 'oop';
	
	private function __construct(){
		$this->db = new \mysqli($this->DB_HOST, $this->DB_USER, $this->DB_PASS, $this->DB_NAME);
		if($this->db->connect_errno > 0){
			throw new \Exception("No connection with database.",500);
		}
	}
	public static function getInstance(){
        if (self::$instance == Null){
            self::$instance = new self;
        }
        return self::$instance;
    }
	
	public function query($sql){
		$query = $this->db->query($sql);
		if($query){
			return $query;
		}
		else{
			throw new \Exception("SQL error.",500);
		}
	}
	
	public function close(){
        if($this->db){
            $this->db->close();
		}
    }
}
?>