class Connection
{
private $username = "xxxx";
private $password = "xxxx";
private $dsn = "mysql:host=host;dbname=name_db";
private $sql;
private $DBH;

function setSQL($sql){
    $this->sql = $sql; //if $sql is not set it troughs an error at PDOException
}

public function query()
{
    //the connection will get established here if it hasn't been already
    if (is_null($this->DBH))
        try {
            $this->DBH = new PDO( $this->dsn, $this->username, $this->password );
            $this->DBH->setAttribute( PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION );

            //query
            $result = $this->DBH->prepare($this->sql);
            $result->execute();
            return $result->fetch(PDO::FETCH_ASSOC);

        } catch(PDOException $e) {

            echo "I'm afraid I can't do that1.";
            //file_put_contents('PDOErrors.txt', $e->getMessage(), FILE_APPEND);
        }
}

//clear connection and variables
function __destruct(){
        $this->DBH = null;
}

}

$sql = "SELECT * from stock WHERE id = 302";
$test = new Connection();
$test->setSQL($sql);
echo $test->query();

class ApplicationResourcePool
{
    static var $_dbHandle;

    private static $_dbConfig = array(
        'dsn'      => '',
        'username' => '',
        'password' => '',

    );
    public static getDbHandle(){
        if(self::$_dbHandle == null){
            self::$_dbHandle = new PDO(
                self::$_dbConfig['dsn'] ,
                self::$_dbConfig['username'],
                self::$_dbConfig['password']
            );
        }
        return self::$_dbHandle;
    }

}

class StockMapper
{
    protected $_dbh;
    public __construct($dbh = null)
    {
        if($dbh == null){
            $this->_dbh = ApplicationResourcePool::getDbHandle();
        } else {
            $this->_dbh = $dbh;
        }

    }
    public getStockById($stockId){

        $sth=$this->_dbh->prepare("SELECT * from stock WHERE id = :stockId");
        $sth->bindParam(":stockId", $stockId);
        $sth->execute();
        $result = $sth->fetch(PDO::FETCH_ASSOC);
        return $result[0];
    }
}

$stockMapper = new StockMapper();
$stockData = $stockMapper->getStockById('302');

SELECT * from stock WHERE id = ?

array(302)