Problem with shirt detail URLs

<code>function get_list_view_html($product_id, $product) {

    $output = "";

    $output = $output . "<li>"; 
    $output = $output . '<a href="' .BASE_URL . 'shirts/' . $product_id . '">';
    $output = $output . '<img src="' . BASE_URL . $product["img"] . '" alt="' . $product["name"] . '">';
    $output = $output . "<p>View Details</p>";
    $output = $output . "</a>";    
    $output = $output . "</li>";

    return $output;
}</code>

Here's my shirt.php file:

<code><?php 
require_once("../includes/config.php");
require_once(ROOT_PATH . "includes/products.php"); 
if (isset($_GET["id"])) {
    $product_id = $_GET["id"];
    if (isset($products[$product_id])) {
        $product = $products[$product_id];
    }
} 
if (!isset($product)) {
    header("Location:" . BASE_URL . "shirts/");
    exit();
}

$section = "shirts";
$pageTitle = $product["name"];
include(ROOT_PATH . "includes/header.php"); ?>

        <div class ="section page">

            <div class="wrapper">

                <div class="breadcrumb"><a href="<?php echo BASE_URL; ?>shirts/">Shirts</a> &gt; <?php echo $product["name"]; ?></div>

                <div class="shirt-picture">
                    <span>
                        <img src="<?php echo BASE_URL . $product["img"]; ?>" alt="<?php echo $pageTitle; ?>"
                    </span>
                </div>

                <div class="shirt-details">

                    <h1><span class="price">$<?php echo $product["price"]; ?></span><?php echo $product["name"]; ?></h1>
                    <p class="note-designer">* All shirts are designed by Mike the Frog. *</p>

                </div>  




            </div>

        </div>

<?php include(ROOT_PATH . "includes/footer.php"); ?></code>