quickie, proper syntax for this?

[listenmirndt]

$_SESSION[cart_items$num] = $number;

this gives me a syntax error,

[bradgrafelman]

The index of an associative array is nothing more than a [man]string[/man], so the basic rules of strings still apply here - either use concatenation or variable interpolation (within strings delimited by double quotes).

EDIT: Also note that I wouldn't suggest using your approach above at all; why not just make $SESSION['car_items'] a numerically-indexed array? That way storing a new item in it is as simple as:

$_SESSION['car_items'][] = $new_item;