<?php
if (isset($_POST ['pid'])) {
    $pid = $_POST ['pid'];
    $wasFound = false;
    $i = 0;
    if (!isset ($_SESSION["cart_array"]) || count($_SESSION ["cart_array"])<1){
        $_SESSION["cart_array"] = array (0=> array("item_id"=> $pid, "quantity" => 1));
    }  else {
        foreach ($_SESSION["cart_array"] as $each_item) {
            $i++;
            while(list($key,$value)=each($each_item)){
            if ($key== "item_id" && $value == $pid) {
                array_splice ($_SESSION["cart_array"], $i-1,1, array(array("item_id"=>$pid,"quantity"=> $each_item['quantity']+1)));
                $wasFound = true;
            }
        }
    }
    if($wasFound==false) {
        array_push ($_SESSION["cart_array"], array("item_id"=> $pid, "quantity" => 1));
        }
    }
    header("Location: cart.php");
    exit();
}
?>

if (isset($_POST['pid'])) {
    // add (+1) item to cart
    $pid = (int)$_POST['pid']; // cast as integer
    // valid pids are > 0
    if($pid > 0){
    if(!isset($_SESSION['cart_array'][$pid])){
    // item is not in the cart, add it with quantity = 1
    $_SESSION['cart_array'][$pid] = array("item_id" => $pid, "quantity" => 1); // I left the array in the cart the same, but it could also be simplified so that it is only the quantity, since the item_id is now the cart array index
    } else {
    // item is in the cart, increment quantity
    $_SESSION['cart_array'][$pid]['quantity']++;
    }
    }
    header("location: cart.php");
    exit();
    }

if  ($result = print_r(array_keys('cart_array',$pid))) {
$comma_seperated = implode("," $result);
// then use use $comma_seperated in query where needed later in annother code block?
}