Shopping cart weird interaction that I don't understand - php

Question

I am creating a shopping cart with an array in php. The array first array is to store the ShoeId and the quantity which was POST from the previous page. It is stored in the $_SESSION['cart'].
I then compared the cart with a new array called $cleanArray to find duplicates and replace it by increasing the quantity. However I came into an issue that I don't understand and after spending countless of hours I decided to ask here.
//creating a final cart
$cleanArray = array();
//to prevent sql injection
$shoeID = $conn->real_escape_string($_POST['shoeID']);
$quantity = $conn->real_escape_string($_POST['quantity']);
// $_SESSION['cart'] = array();
If I leave the $_SESSION['cart'] = array(); in the code the cart will keep clearing itself.
However, If i add it, refresh my webpage and then remove that code again. My shopping cart would work as intended.
//checking if Cart Array exist and if ShoeID and quantity has been selected
if (isset($_POST['shoeID']) && ($_POST['quantity'])){
if(isset($_SESSION['cart'])){
echo "shoeID: $shoeID";
echo "<br>";
echo "Quantity: $quantity";
echo "<br>";
$_SESSION['cart'][] = array('shoeId' => $shoeID, 'quantity' => $quantity);
$cartArray = $_SESSION['cart'];
//find for duplicate shoeId and then increase the quantity.
foreach($cartArray as $item){
if(isset($cleanArray[$item['shoeId']])){
$cleanArray[$item['shoeId']]['quantity'] += $item['quantity'];
} else {
$cleanArray[$item['shoeId']] = $item;
}
}
} else {
echo "Cart Is Empty!";
}
$cleanArray = array_values($cleanArray);
print_r($cleanArray);
$_SESSION['finalCart'] = $cleanArray;
Any forms of suggestion would be helpful as I am new to php and I am trying to understand more about it. thank you!