How can I pass a variable from a form through POST?

Question

Now I know how to do it but I keep getting an error saying the index is undefined. A little about what I'm trying to do: I have a cart and I want to update the quantities of items in the cart. Currently my quantity is updated as one clicks on the add item button. But as soon as I try to update the quantity by hand through a form I get an error saying the index I'm trying to pass is undefined. As I am pretty new to php I don't really know how or why it's not being passed with the value I set it to.

Now for the code:

This is my cart (the part that doesn't work properly):

<?php
            if (isset($_SESSION["cart"])) {
                $total = 0;
                foreach(BazaKnjig::seznamVsehKnjig() as $knjiga):
                    if (isset($_SESSION["cart"][$knjiga->id])) { 
                        $number = $_SESSION["cart"][$knjiga->id];    
                        
                        ?>
                        <form action="<?= $url ?>" method="post">
                            <input type="hidden" name="do" value="update_cart" />
                            <input type="hidden" name="id" value="<?= $knjiga->id ?>" />
                            <p>
                                <input type="number" name="numBooks" value="<?= $number ?>" size="1">  <?= $knjiga->avtor ?>: <?= substr($knjiga->naslov, 0, 20) ?> ...
                                <button type="submit">Posodobi</button>
                            </p>
                        </form>
                        <?php
                            $total = $total + ($knjiga->cena) * $number;
                    }
                endforeach;
                ?>
                
               <p> Skupaj: <b> <?= number_format($total, 2) ?> EUR </b></p>

I have a form to present all of the items in the cart. And when passing the variables with POST I get an error Notice: Undefined index: numBooks . I did some googling and found that this is common when the value of the variable is empty. I am not sure if this has something to do with the fact that I'm setting the quantity with the variable $number or if I'm missing something crucial.

And here is my code for checking the method and filtering.

if ($method == "POST") {
    $validationRules = [
        'do' => [
            'filter' => FILTER_VALIDATE_REGEXP,
            'options' => [
                "regexp" => "/^(add_into_cart|purge_cart|update_cart)$/"
            ]
        ],
        'id' => [
            'filter' => FILTER_VALIDATE_INT,
            'options' => ['min_range' => 0]
        ]
    ];
    $post = filter_input_array(INPUT_POST, $validationRules);

Then follow switch statements for the other functionalities so I will skip these as they aren't problematic and finally the update switch case:

switch($post["do"]):
     case "update_cart":
            $id = $post["id"];
            $knjiga = BazaKnjig::vrniKnjigo($id);
            $quan = $post["numBooks"];
            
            if($quan > 0) {
                $_SESSION["cart"][$id] = $quan; 
            } else {
                unset($_SESSION["cart"][$id]);
            }
            break;

The error appears at $quan = $post["numBooks"]; . Any help is appreciated.

Answer 1

根据 Mihai 的建议，我为“numBooks”添加了一个额外的过滤器，从而解决了问题。

Answer 2

there is no

$quan = $post["numBooks"]

in your code, add that.