Ask about the php code.

 function get_product_single($sku){
    require(ROOT_PATH. "inc/database.php");
    try{
        $results=$db->prepare("SELECT name, price, img, sku, paypal FROM products where sku=?");
        $results->bindParam(1,$sku);
        $results->execute();
    }catch(Exception$e){
      echo "Data could not be retried from the database.";
      exit;
    }

    $product=$results->fetch(PDO::FETCH_ASSOC);
    if($product===false)return$product;
    $product["sizes"] = array();
    try{
        $results=$db->prepare("
        SELECT size
        FROM products_sizesps 
        INNER JOIN SIZES ON ps.size_id=s.id
        WHERE product_sku=?
        ORDER BY `order`);
        $results->bindParam(1,$sku);
        $results->execute();
    } catch(Exception$e){
        echo "Data could not be retried from the database.";
        exit;
    }
    while($row=$results->fetch(PDO::FETCH_ASSOC)){
        $product["sizes"][ ] = $row["size"];
    }

    return$product;
}