让我先给你代码，并在下面解释：

<?php
$productID = $_POST['productID'];
$productAmount = $_POST['productAmount'];
//Declare variables
$cartNumItems = 0;
//Check if cookie exists and set variable if it does
if(isset($_COOKIE['cart'])){
    if($_COOKIE['cart'] != null && $_COOKIE['cart'] != ''){
        $cart = $_COOKIE['cart'];
    }
}
//Get array from cookie
if(isset($cart)){
    //Decode array from $cart variable and store it
    $cartArray = json_decode($cart);
    //Count number of items in $cart
    $cartAmount = count($cartArray);
    for($i = 0; $i < $cartAmount; $i++){
        //Check if $cart has the same product in it already
        if($productID == $cartArray[$i][0]){
            //Add to the amount of that product
            $cartArray[$i][1] += $productAmount;
            break;
        }
        //If it does not have the same product already, just add a new one
        else if($i == ($cartAmount - 1)){
            array_push($cartArray, array($productID, $productAmount));
            break;  
        };
    };
    //Recount number of items in $cart
    for($i = 0; $i < $cartAmount; $i++){
        $cartNumItems += $cartArray[$i][1];
    };
    //Encode $cart so it can be stored in cookie
    $cartRaw = json_encode($cartArray);
    //Create cookies
    setcookie('cart', $cartRaw, time() + (86400 * 7), '/');
    setcookie('cartNumItems', $cartNumItems, time() + (86400 * 7), '/');
    echo 'true';
}
else{
    //Create the info that needs to be put into cookie
    $cart = json_encode(
        array(
            array($productID, $productAmount)
        )
    );
    $cartArray = json_decode($cart);
    //Count and store the amount of items in cart array
    $cartAmount = count($cartArray);
    //Store amount of items in $cartNumItems variable
    for($i = 0; $i < $cartAmount; $i++){
        $cartNumItems += $cartArray[$i][1];
    };
    //Create cookies
    setcookie('cart', $cart, time() + (86400 * 7), '/');
    setcookie('cartNumItems', $cartNumItems, time() + (86400 * 7), '/');
    echo 'true';
};
?>

第一次运行此代码时，不会设置"购物车"cookie。因此，它在检查变量"$cart"是否设置时运行 big else 语句。else 语句工作正常，并执行它应该执行的操作。它创建一个 2D 数组，然后使用所需信息创建 cookie。

代码第二次运行时，应运行 if 语句。我相信确实如此。"购物车"cookie（又名"$cart"变量）设置为 null。然后将"cartNumItems"cookie（又名"$cartNumItems"变量）设置为 0。这是不应该做的。我只是找不到它在哪里这样做，或者为什么这样做。

如果对具体是什么问题代码有任何混淆，这就是问题代码（看起来）：

//Get array from cookie
if(isset($cart)){
    //Decode array from $cart variable and store it
    $cartArray = json_decode($cart);
    //Count number of items in $cart
    $cartAmount = count($cartArray);
    for($i = 0; $i < $cartAmount; $i++){
        //Check if $cart has the same product in it already
        if($productID == $cartArray[$i][0]){
            //Add to the amount of that product
            $cartArray[$i][1] += $productAmount;
            break;
        }
        //If it does not have the same product already, just add a new one
        else if($i == ($cartAmount - 1)){
            array_push($cartArray, array($productID, $productAmount));
            break;  
        };
    };
    //Recount number of items in $cart
    for($i = 0; $i < $cartAmount; $i++){
        $cartNumItems += $cartArray[$i][1];
    };
    //Encode $cart so it can be stored in cookie
    $cartRaw = json_encode($cartArray);
    //Create cookies
    setcookie('cart', $cartRaw, time() + (86400 * 7), '/');
    setcookie('cartNumItems', $cartNumItems, time() + (86400 * 7), '/');
    echo 'true';
}

更新：

我已经找到了重置cookie的东西，if语句中的这段代码：

//Decode array from $cart variable and store it
$cartArray = json_decode($cart);

当它运行时，$cartArray设置为 null，因此代码的其余部分正在处理 null 变量。现在我不明白的是为什么它被设置为 null。当我var_dump$cart变量时，我得到字符串（16）"[[''"21''"，''"1''"]]"。但是当我跑步时...

echo json_decode($cart);

它只是空白的。