codice:
$_SESSION['carrello'] = '1,5,341,249,12,34,689';

$array = explode(',', $_SESSION['carrello']);
foreach($array as $value) {
echo $value;
}

//oppure...

list($v1, $v2, $v3, $v4, $v5, $v6, $v7) =
explode(',', $_SESSION['carrello']);

echo $v1;
echo $v2;
//echo $v3; etc...