Carrello in PHP

[satifal]

session_start();
include("config.php");
$id_art = mysql_real_escape_string($_GET['id_art']);
if (empty($_SESSION['carrello'])) {
    $_SESSION['carrello'] = $id_art;
} else {
    $_SESSION['carrello'] .= ",".$id_art;
}
echo $_SESSION['carrello'];