PHP Session problem. When item is added it adds 1 to all of the items

Question

I am making a shopping cart where when you click on either of two cars you add that item to the shopping cart. The problem is that when I click for example on the BMW picture the count goes up for both BMW and Toyota. I've been struggling on this for over 2 days now and I can't figure it out!

Here is my first page. When I click on either of pictures I want it to add it to the shopping cart. I want to use session to work this out.

    <?php session_start(); ?>
    <?php include('navbar.php');?>

    <h1>OstosKoriTori</h1>
    <h3>Click the picture to add the car to your shopping cart!</h3>
    <p>Toyota Chaser JZX 100</p>
    <p>BMW E92</p>
    <div class="autot">

    <a href="add-basket.php?id=chaser"><img src="chaser.jpg" alt="Chaseri"></a>
    <a href="add-basket.php?id=bmw"><img src="bmw.jpg" alt="Bemari"></a>
    <br>
    </div>

Here is the second page, this code is run when you select the picture. I believe the problem is here.

    <?php session_start(); ?>
    <?php
    if(!isset($_SESSION['chaser'])){
    $_SESSION['chaser'] = 0;
    }
    if(isset($_SESSION['chaser'])){
    $_SESSION['chaser'] += 1;
    }
    if(!isset($_SESSION['bmw'])){
    $_SESSION['bmw'] = 0;
    }
    if(isset($_SESSION['bmw'])){
    $_SESSION['bmw'] += 1;
    }
    ?>
    <?php
    header("Location: http://" . $_SERVER['HTTP_HOST']
    . dirname($_SERVER['PHP_SELF']) . '/'
    . "basket-session.php"); 
    ?>

And here is the last page where you can see how many cars you have depending on how many times you clicked on the picture.

    <?php session_start(); ?>
    <?php include('navbar.php')?>
    <h1>OstosKoriTori</h1>
    <h3>Ostoskorin sisältö</h3>
    <p>Toyota Chaser JZX 100</p>
    <p>BMW E92</p>
    <div class="kuvat">
    <img src="chaser.jpg" alt="Chaseri">
    <img src="bmw.jpg" alt="Bemari">
    </div>
    <div class="kpl">
    <p><?php echo "Korissa on: <strong>" . $_SESSION['chaser'] . "</strong> kpl"; ?></p>
    <p><?php echo "Korissa on: <strong>" . $_SESSION['bmw'] . "</strong> kpl"; ?></p>
    </div>
    <div class="clearBasket">
    <a href="clear-basket.php">Tyhjennä ostoskori</a>
    </div>

Answer 1

Based on the code you've posted, something like this should work. I've added some comments to explain the code.

<?php 

session_start();

// Initialise session to existing value or zero
$_SESSION['chaser'] = $_SESSION['chaser'] ?? 0;
$_SESSION['bmw'] = $_SESSION['bmw'] ?? 0;

// Determine selected car
$car = $_GET['id'] ?? null;

// Increment selected car
if ($car) {
    $_SESSION[$car] += 1;
}

header('Location: http://' . $_SERVER['HTTP_HOST'] . dirname($_SERVER['PHP_SELF']) . '/basket-session.php');