How come checkbox state is not always passed along to PHP script?

If you want to overcome this design feature, try this:

<input type="hidden" name="check_box_1" value="0" />
<input type="checkbox" name="check_box_1" value="1" />

This way, you'll always have $_POST['check_box_1'] set in the callback page, and then you can just check its value to see whether they checked it or not. The two inputs have to be in that order though, since the later one will take precedence.

Give the checkboxes names as array like

<input type = "checkbox" value = "box" name = "checkbox[]"/>

And after submit try like

$checked_arr = $_POST['checkbox'];
$count = count($checked_arr);
echo "There are ".$count." checkboxe(s) are checked";

Note : And based on the method that your form submit using...whether it is $_GET or $_POST you need to use $_POST['checkbox'] for POST method and $_GET['checkbox'] for the GET method.

Three words: array it up!

Observe this code:

<form action="order.php" method="post">
    <p>
        Coffee: <br>

        <!-- This ensures false is submitted if the cappuccino box is unticked -->
        <input type="hidden" name="coffee[cappuccino][selected]" value="0">
        <label><input type="checkbox" name="coffee[cappuccino][selected]" value="1"> Cappuccino</label>
        <input type="number" name="coffee[cappuccino][qty]" size="2"><br>

        <!-- This ensures false is submitted if the espresso box is unticked -->
        <input type="hidden" name="coffee[espresso][selected]" value="0">
        <label><input type="checkbox" name="coffee[espresso][selected]" value="1"> Espresso</label>
        <input type="number" name="coffee[espresso][qty]" size="2"><br>
    </p>

    <p>[...]</p>

    <p>
        <label><input type="radio" name="dine" value="in"> Dine in</label>
        <label><input type="radio" name="dine" value="out"> Take out</label>
    </p>

    <p><input type="submit" value="submit" name="submit"></p>
</form>

When this submits, the inputs are submitted as an associative array as this:

array (size=3)
  'coffee' => 
    array (size=2)
      'cappuccino' => 
        array (size=2)
          'selected' => string '1' (length=1)
          'qty' => string '4' (length=1)
      'espresso' => 
        array (size=2)
          'selected' => string '1' (length=1)
          'qty' => string '3' (length=1)
  'dine' => string 'in' (length=2)
  'submit' => string 'submit' (length=6)

As you can see, each coffee is now being submitted as an array with the type of coffee being a key that also has an array of its own with two further keys: selected and qty.

The selected determines if the checkbox for that coffee was ticked (1) or not (0) and the qty holds the user input.

This pretty much does your validation since each quantity input now belongs to the individual coffee.

I hope I've understood your dilemma correctly and this answers your question or give a you an idea of how to go on about doing this.

Enjoy :)