build-cart-items.php

<?php
# Final storage for the products our script 
# will send to BlackPay for billing
$cartItems = array(); # or [], who cares?

# Get an array of all items the customer has 
# placed in the shopping basket
$basket = $this->getShoppingBasketContents('basket_uuid');

# If the previous array has extra information, you can 
# filter it out so you are left with the required 
# fields only as specified in the docs
foreach($basket as $key => $item){
    
    # Make empty array to hold the required fields only
    $tmp = [
        'name'  =>  $item['name'], // Product name
        'qty'   =>  $item['quantity'], // How many?
        'price'   =>  $item['price'], // Price per item
    ];
    
    # Push each product to $cartItems
    $cartItems[] = $tmp;
}

# Encode to JSON string. Recommended to eliminate 
# all whitespace from the JSON string.
$cartItems = json_encode($cartItems);

# Encode JSON string to BASE64 string for easy 
# transport to BlackPay Express
$cartItems = base64_encode($cartItems);
?>

<!--Send your request with an normal POST payload-->
<form method="POST" action="[ENTER BLACKPAY EXPRESS ENDPOINT]">
    <input type="hidden" name="store_id" value="[ENTER STORE ID]">
    <input type="hidden" name="cart_id" value="[CART UNIQUE ID]">
    <input type="hidden" name="cart_items" value="<?php echo $cartItems?>"> <!-- $cartItems string here -->
    <input type="hidden" name="grand_total" value="[GRAND TOTAL]">
    <input type="hidden" name="success_url" value="[LINK TO REDIRECT ON SUCCESS]">
    <input type="hidden" name="failed_url" value="[LINK TO REDIRECT ON FAIL]">
    <input type="hidden" name="cancel_url" value="[LINK TO REDIRECT ON CANCEL]">
    <input type="hidden" name="customer_email" value="[CUSTOMER EMAIL]">
    <button type="submit" class="btn btn-success btn-lg btn-block">Pay with M-PESA</button>
</form>