class DataBaseConnection {
    static private $instance = NULL;
    private $objMySqli;
    private function __construct() {
        $this->objMySqli=new mysqli("host","user","passwd","database_name");
    }
    static public function getInstance() {
       if (self::$instance == NULL) self::$instance = new self;
       return self::$instance;
    }
    public function __clone()
    {
        trigger_error('Clone is not allowed.', E_USER_ERROR);
    }
    public function getMySqli(){
        return $this->objMySqli;
    }
}
//Usage
$connection= DataBaseConnection::getInstance();//getting the instance forom of the class DataBaseConnection
$result = $connection->getMySqli()->query("select * from tablename");
while($row=$result->fetch_row()){
    //print the data of the $row variable
}
$connection2 = new DataBaseConnection()//this will produce an error