First of all, this link ...
<a href="artificial_christmas_tree_details.php?product_name=<?php echo ['$row_prod['product_name']; ?>
... is not well-formed as it stands. Should be something like ...
<a href="artificial_christmas_tree_details.php?product_name=<?php echo ['$row_prod['product_name']; ?>"><?php echo ['$row_prod['product_name']; ?></a>
Secondly, how is product_name referred to in artificial_christmas_tree_details.php? You should use $GET['product_name'] and not just $product_name. Using the $GET (or $_POST) variable is now default behaviour.
(BTW Don't use $HTTP_GET_VARS['product_name'] either ... this has been deprecated altogether).
Paul 🙂
PS. The %20 is just URL-encoded 'space' and shouldn't be a problem.