$_POST on the same document as the form

Question

I have a problem with html forms and PHP on the same page. This is a part of the code I just wrote:

Here is the form where I want to insert the ID in an array by clicking the submit button. When I submit the form, the value for menge is always empty. Does somebody know why this problem occurs?

echo '<form name="aanzahl" method="POST" action="index.php?page=einzelansicht&id='.$a_id.'">';
echo '<input id="inputButton" type="text" name="menge" value="1" size="2" maxlength="2" disabled>';
echo '<input type="submit"  class ="warenkorbbtn" name="warenkorbbtn" value="In den Warenkorb">';
echo '</form>';

if(isset($_POST['warenkorbbtn'])){
    if (!isset($_SESSION['warenkorb'])){
        $_SESSION['warenkorb'] = array();
    }

    $_SESSION['warenkorb'][] = array('warenkorb_id'=>$a_id, 'menge'=>$_POST['menge']);
    echo "Artikel befindet sich nun im Warenkorb";
}

Answer 1

A form element with attribute disabled will not have its value sent with the form:

This Boolean attribute indicates that the form control is not available for interaction. In particular, the click event will not be dispatched on disabled controls. Also, a disabled control's value isn't submitted with the form.¹

In order to have the value sent with the form yet still prevent the user from modifying the value, use the attribute readonly (though nothing can prevent the user from altering/removing that attribute with their browser console).

<input id="inputButton" type="text" name="menge" value="1" size="2" maxlength="2" readonly />

And if you don't care about the user even seeing the input, you could make the input type hidden (i.e. (input type="hidden" />), though the user could still change the value using their browser console.

You can see the readonly property demonstrated with this phpfiddle, where the readonly attribute has been set on that input.

Also note that an input element has no permitted content and thus is an [empty element], so I added the slash to the end of the element in order to close it properly (i.e. <input..../>).

In HTML, using a closing tag on an empty element is usually invalid. For example, is invalid HTML.²

Answer 2

If you set the disabled attribute to an input element then the value of that element is not sent when you fire the "submit" action. Try to remove the disabled attribute to get the field value.