TWO CLASSES
class USER
{
    private $db;

    function __construct($DB_con)
    {
      $this->db = $DB_con;
    }

class FORUM {

	function __construct($DB_con)
    {
      $this->db = $DB_con;
    }

	public function getSubject() {
		$table = "";
		try {
		$query = $this->db->prepare("DESCRIBE posts");
		$query->execute();
		$forumrow = $query->fetchall(PDO::FETCH_COLUMN);
		print_r($forumrow);
		} catch(PDOException $e) {
			echo $e->getMessage();
		}
	}
}

$forum = new FORUM($DB_con);
$forum->getSubject();

I have a database conntection too.

session_start();

$DB_host = "localhost";
$DB_user = "root";
$DB_pass = "";
$DB_name = "dblogin";

try
{
     $DB_con = new PDO("mysql:host={$DB_host};dbname={$DB_name}",$DB_user,$DB_pass);
     $DB_con->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
}
catch(PDOException $e)
{
     echo $e->getMessage();
}


include_once 'users.php';
$user = new USER($DB_con);