The point of this question

What about this piece of code? I'm actually using it while developing a simple application and it seems to cover all my needs. Also it uses PDO so that we don't really have to worry about SQL Injection. I know i usually code strange, but i hope you could give me suggestions and feedback in order to improve it. Thanks.

Code: Database Class

/* Operate on the database using our super-safe PDO system */
class db
{
    /* PDO istance */
    private $db = NULL;
    /* Number of the errors occurred */
    private $errorNO = 0;

    /* Connect to the database, no db? no party */
    public function __construct()
    {
        try
        {
            $this->db = new PDO(
                'mysql:dbname='.reg::get('db-name').';host='.reg::get('db-host'), reg::get('db-username'), reg::get('db-password')
            );
        }
        catch (Exception $e) 
        {
            exit('App shoutdown');
        }
    }
    
    /* Have you seen any errors recently? */
    public function getErrors() { return ($this->errorNO > 0) ? $this->errorNO : false; }
    
    /* Perform a full-control query */
    public function smartQuery($array)
    {
        # Managing passed vars
        $sql = $array['sql'];
        $par = (isset($array['par'])) ? $array['par'] : array();
        $ret = (isset($array['ret'])) ? $array['ret'] : 'res';
        
        # Executing our query
        $obj = $this->db->prepare($sql);
        $result = $obj->execute($par);
        
            # Error occurred...
            if (!$result) { ++$this->errorNO; }
        
        # What do you want me to return?
        switch ($ret)
        {
            case 'obj':
            case 'object':
                return $obj;
            break;
            
            case 'ass':
            case 'assoc':
            case 'fetch-assoc':
                return $obj->fetch(PDO::FETCH_ASSOC);
            break;
            
            case 'all':
            case 'fetch-all':
                return $obj->fetchAll();
            break;
            
            case 'res':
            case 'result':
                return $result;
            break;
            
            default:
                return $result;
            break;
        }
    }
    
    /* Get PDO istance to use it outside this class */
    public function getPdo() { return $this->db; }
    
    /* Disconnect from the database */
    public function __destruct() { $this->db = NULL; }
}

Use

$db = new db;
$user = $db->smartQuery(array(
    'sql' => "SELECT UserName FROM `post` WHERE UserUID = :uid",
    'par' => array('uid' => $uid),
    'ret' => 'fetch-assoc'
));
echo $user['Username'];