Drupal 7: Ubercart - Change the value of "Add to cart" when a product is already in a user's cart, or the product is out of stock.

d7_out_of_stock_remove_addtocart.php

<?php
    // Get the "Add to cart" array ready for rendering.
    $add_to_cart = array(
        '#theme' => 'uc_product_add_to_cart',
        '#form' => drupal_get_form('uc_product_add_to_cart_form_' . $node->nid, $node),
    );
    /*
    * Check the database to see the Stock availability of the product.
    */
    if(uc_stock_is_active($node->model)){
        $stock = uc_stock_level($node->model);
    }
    /*
    * Check whether or not this product is already in the current
    * user's cart. 
    */
    $already_added = FALSE; 
    $cart = uc_cart_get_contents();
    foreach(element_children($cart) as $child){
        if($cart[$child]->nid == $node->nid){
            $already_added = TRUE;    
        }
    }
    /*
    * If the Stock value is not 0 and the item is not in the current
    * user's cart, render the "Add to cart" form normally.
    *
    * If the item is already in the user's cart, disable the button
    * and replace the message. Products on this site are unique.
    *
    * If the Stock value is less than 1, disable the button and
    * replace the message. This product has been sold out.
    */
    if($stock != 0 && $already_added == FALSE){
        print drupal_render($add_to_cart);
    }elseif($already_added == TRUE){
        print '<input class="node-add-to-cart form-submit" type="submit" name="op" disabled value="Already in your cart!">'; 
    }elseif($stock < 1){
        print '<input class="node-add-to-cart form-submit" type="submit" name="op" disabled value="Sold Out">';  
    }

    /*
    * A great compiment to this is the uc_out_of_stock module on drupal.org
    * which uses Ajax to render messages to the user when they attempt
    * to add more products to their cart than there is stock available.
    *
    * http://drupal.org/project/uc_out_of_stock
    */
?>