Hi,
I do have lots of PHP/MySQL experience but never implemented a basket/cart, so I can't judge your overall approach. However I do have several comments:
1.
General comment: too much effort and code have been spent early on presentation while the code is not yet performing essential functions correctly. My advice is: get it to work first (with as few lines of code as possible, no bootstrap, no styles) and only when all technical problems have been solved then take care of layout, style, and minor details.
2.
In your statement
<form action="cart.php?broodjes_ID='. $row['broodjes_ID'].'" method ...
(a) you are mixing HTML and PHP without using <?php and ?>
(b) you are missing an equal sign after "value"
(c) however value is not an official parameter for a form; what happened?
(d) you are passing broodjes_ID as a URL parameter while specifying a post method. I'm not sure that will ever work. Normal way of passing data with forms AFAIK is by using hidden fields. Something like
$brID=$row['broodjes_ID'];
echo "<input type='hidden' name='brID' value='$brID'>";
3.
You have
$_SESSION['basket'][] = $row['broodjes_ID'];
inside the while loop that iterates all broodjes; this means 'basket' is an array and one broodje is to be appended to the array, so I'm not surprised you are always ordering one of everything.
4.
Suggestion:
clean up your code, remove code that got commented out, do keep comments that explain what you intend to do and why you are doing it in the chosen way. Also take care of indentation. Clean code helps in understanding what is going on.