Hi everyone,
This question has been raised before on this forum, but I have been unable to find an answer.
I am trying to add the quantity discount box from the product info page to the product listing page. I have tried every possible combination under the sun, and I feel that the closer I get, the further away I am.
I am modifying 2 files: product_listing.php and tpl_modules_products_quantity_discounts.php. I am using the template override system.

My idea is to alter the value of $lc_price to contain the HTML for the quantity discount box.

Here are the files, so you can have a look:

tpl_modules_products_quantity_discounts.php
PHP Code:
<?php
/**
 * Module Template
 *
 * @package templateSystem
 * @copyright Copyright 2003-2005 Zen Cart Development Team
 * @copyright Portions Copyright 2003 osCommerce
 * @license http://www.zen-cart.com/license/2_0.txt GNU Public License V2.0
 * @version $Id: tpl_modules_products_quantity_discounts.php 2008-02-04 PRedmond $
 * Based on: tpl_modules_products_quantity_discounts.php 3291 2006-03-28 04:03:38Z ajeh $
 */
 
  
if ($zc_hidden_discounts_on) {
   
$lc_price '<table border="1" cellspacing="2" cellpadding="2">';
   
$lc_price .= '<tr>';
   
$lc_price .= '<td colspan="1" align="center">';
   
$lc_price .= TEXT_HEADER_DISCOUNTS_OFF;
   
$lc_price .= '</td>';
   
$lc_price .= '</tr>';
   
$lc_price .= '<tr>';
   
$lc_price .= '<td colspan="1" align="center">';
   
$lc_price .= $zc_hidden_discounts_text;
   
$lc_price .= '</td>';
   
$lc_price .= '</tr>';
   
$lc_price .= '</table>';
 } else {
$lc_price .= '<table border="1" cellspacing="2" cellpadding="2">';
$lc_price .= '<tr>';
$lc_price .= '<td colspan="' . ($columnCount+1) . '" align="center">';
  switch (
$products_discount_type) {
    case 
'1':
     
$lc_price .= TEXT_HEADER_DISCOUNT_PRICES_PERCENTAGE;
      break;
    case 
'2':
      
$lc_price .= TEXT_HEADER_DISCOUNT_PRICES_ACTUAL_PRICE;
      break;
    case 
'3':
      
$lc_price .= TEXT_HEADER_DISCOUNT_PRICES_AMOUNT_OFF;
      break;
  }
$lc_price .= '</td>';
$lc_price .= '</tr>';
$lc_price .= '<tr>';
$lc_price .= '<td align="center">' $show_qty '<br />' $currencies->display_price($show_pricezen_get_tax_rate($products_tax_class_id)) . '</td>';
 
  foreach(
$quantityDiscounts as $key=>$quantityDiscount) {
$lc_price .= '<td align="center">' $quantityDiscount['show_qty'] . '<br />' $currencies->display_price($quantityDiscount['discounted_price'], zen_get_tax_rate($products_tax_class_id)) . '</td>';
    
$disc_cnt++;
    if (
$discount_col_cnt == $disc_cnt && !($key == sizeof($quantityDiscount))) {
      
$disc_cnt=0;
$lc_price .= '</tr><tr>';
    }
  }
  if (
$disc_cnt $columnCount) {
$lc_price .= '<td align="center" colspan="' . ($columnCount+$disc_cnt)+'> &nbsp; </td>';
 } 
$lc_price .= '</tr>';
  if (
zen_has_product_attributes($products_id_current)) {
$lc_price .= '<tr>';
$lc_price .= '<td colspan="' $columnCount+'" align="center">';
$lc_price .= TEXT_FOOTER_DISCOUNT_QUANTITIES
$lc_price .= '</td>';
$lc_price .= '</tr>';
 
  } 
$lc_price .= '</table>';
 
  } 
// hide discounts

product_listing.php
PHP Code:
<?php
/**
 * product_listing module
 *
 * @package modules
 * @copyright Copyright 2003-2006 Zen Cart Development Team
 * @copyright Portions Copyright 2003 osCommerce
 * @license http://www.zen-cart.com/license/2_0.txt GNU Public License V2.0
 * @version $Id: product_listing.php 4655 2006-10-02 01:02:38Z ajeh $
 */
if (!defined('IS_ADMIN_FLAG')) {
  die(
'Illegal Access');
}
$show_submit zen_run_normal();
$listing_split = new splitPageResults($listing_sqlMAX_DISPLAY_PRODUCTS_LISTING'p.products_id''page');
$how_many 0;
$list_box_contents[0] = array('params' => 'class="productListing-rowheading"');
$zc_col_count_description 0;
$lc_align '';
for (
$col=0$n=sizeof($column_list); $col<$n$col++) {
  switch (
$column_list[$col]) {
    case 
'PRODUCT_LIST_MODEL':
    
$lc_text TABLE_HEADING_MODEL;
    
$lc_align '';
    
$zc_col_count_description++;
    break;
    case 
'PRODUCT_LIST_NAME':
    
$lc_text TABLE_HEADING_PRODUCTS;
    
$lc_align '';
    
$zc_col_count_description++;
    break;
    case 
'PRODUCT_LIST_MANUFACTURER':
    
$lc_text TABLE_HEADING_MANUFACTURER;
    
$lc_align '';
    
$zc_col_count_description++;
    break;
    case 
'PRODUCT_LIST_PRICE':
    
$lc_text TABLE_HEADING_PRICE;
    
$lc_align 'right' . (PRODUCTS_LIST_PRICE_WIDTH '" width="' PRODUCTS_LIST_PRICE_WIDTH '');
    
$zc_col_count_description++;
    break;
    case 
'PRODUCT_LIST_QUANTITY':
    
$lc_text TABLE_HEADING_QUANTITY;
    
$lc_align 'right';
    
$zc_col_count_description++;
    break;
    case 
'PRODUCT_LIST_WEIGHT':
    
$lc_text TABLE_HEADING_WEIGHT;
    
$lc_align 'right';
    
$zc_col_count_description++;
    break;
    case 
'PRODUCT_LIST_IMAGE':
    
$lc_text TABLE_HEADING_IMAGE;
    
$lc_align 'center';
    
$zc_col_count_description++;
    break;
  }
  if ( (
$column_list[$col] != 'PRODUCT_LIST_IMAGE') ) {
    
$lc_text zen_create_sort_heading($_GET['sort'], $col+1$lc_text);
  }
 
  
$list_box_contents[0][$col] = array('align' => $lc_align,
                                      
'params' => 'class="productListing-heading"',
                                      
'text' => $lc_text );
}
if (
$listing_split->number_of_rows 0) {
  
$rows 0;
  
$listing $db->Execute($listing_split->sql_query);
  
$extra_row 0;
  while (!
$listing->EOF) {
    
$rows++;
    if (((
$rows-$extra_row)/2) == floor(($rows-$extra_row)/2)) {
      
$list_box_contents[$rows] = array('params' => 'class="productListing-even"');
    } else {
      
$list_box_contents[$rows] = array('params' => 'class="productListing-odd"');
    }
    
$cur_row sizeof($list_box_contents) - 1;
    for (
$col=0$n=sizeof($column_list); $col<$n$col++) {
      
$lc_align '';
      switch (
$column_list[$col]) {
        case 
'PRODUCT_LIST_MODEL':
        
$lc_align '';
        
$lc_text $listing->fields['products_model'];
        break;
        case 
'PRODUCT_LIST_NAME':
        
$lc_align '';
        if (isset(
$_GET['manufacturers_id'])) {
          
$lc_text '<h3 class="itemTitle"><a href="' zen_href_link(zen_get_info_page($listing->fields['products_id']), 'products_id=' $listing->fields['products_id']) . '">' $listing->fields['products_name'] . '</a></h3><div class="listingDescription">' zen_trunc_string(zen_clean_html(stripslashes(zen_get_products_description($listing->fields['products_id'], $_SESSION['languages_id']))), PRODUCT_LIST_DESCRIPTION) . '</div>' ;
        } else {
          
$lc_text '<h3 class="itemTitle"><a href="' zen_href_link(zen_get_info_page($listing->fields['products_id']), ($_GET['cPath'] > 'cPath=' $_GET['cPath'] . '&' '') . 'products_id=' $listing->fields['products_id']) . '">' $listing->fields['products_name'] . '</a></h3><div class="listingDescription">' zen_trunc_string(zen_clean_html(stripslashes(zen_get_products_description($listing->fields['products_id'], $_SESSION['languages_id']))), PRODUCT_LIST_DESCRIPTION) . '</div>';
        }
        break;
        case 
'PRODUCT_LIST_MANUFACTURER':
        
$lc_align '';
        
$lc_text '<a href="' zen_href_link(FILENAME_DEFAULT'manufacturers_id=' $listing->fields['manufacturers_id']) . '">' $listing->fields['manufacturers_name'] . '</a>';
        break;
        case 
'PRODUCT_LIST_PRICE':
 
 
    
$products_discount_type zen_products_lookup($listing->fields['products_id'], $what_field 'products_discount_type');
    if (
$products_discount_type != 0) {
      
$rows++;
      
$products_id_current = (int)$listing->fields['products_id'];
      
$quantityDiscounts = array();
      
$products_discount_type_from zen_products_lookup($listing->fields['products_id'], $what_field 'products_discount_type_from');
      require(
DIR_WS_MODULES zen_get_module_directory(FILENAME_PRODUCTS_QUANTITY_DISCOUNTS));
      require(
$template->get_template_dir('/tpl_modules_products_quantity_discounts.php',DIR_WS_TEMPLATE$current_page_base,'templates'). '/tpl_modules_products_quantity_discounts.php');
  }
 
          echo 
$lc_price;
        
//$lc_price = zen_get_products_display_price($listing->fields['products_id'])  . " OK";
        
$lc_align 'right';
        
$lc_text =  $lc_price;
 
 
        
// more info in place of buy now
        
$lc_button '';
        if (
zen_has_product_attributes($listing->fields['products_id']) or PRODUCT_LIST_PRICE_BUY_NOW == '0') {
          
$lc_button '<a href="' zen_href_link(zen_get_info_page($listing->fields['products_id']), ($_GET['cPath'] > 'cPath=' $_GET['cPath'] . '&' '') . 'products_id=' $listing->fields['products_id']) . '">' MORE_INFO_TEXT '</a>';
        } else {
          if (
PRODUCT_LISTING_MULTIPLE_ADD_TO_CART != 0) {
            if (
                
// not a hide qty box product
                
$listing->fields['products_qty_box_status'] != &&
                
// product type can be added to cart
                
zen_get_products_allow_add_to_cart($listing->fields['products_id']) != 'N'
                
&&
                
// product is not call for price
                
$listing->fields['product_is_call'] == 0
                
&&
                
// product is in stock or customers may add it to cart anyway
                
($listing->fields['products_quantity'] > || SHOW_PRODUCTS_SOLD_OUT_IMAGE == 0) ) {
              
$how_many++;
            }
            
// hide quantity box
            
if ($listing->fields['products_qty_box_status'] == 0) {
              
$lc_button '<a href="' zen_href_link($_GET['main_page'], zen_get_all_get_params(array('action')) . 'action=buy_now&products_id=' $listing->fields['products_id']) . '">' zen_image_button(BUTTON_IMAGE_BUY_NOWBUTTON_BUY_NOW_ALT'class="listingBuyNowButton"') . '</a>';
            } else {
              
$lc_button TEXT_PRODUCT_LISTING_MULTIPLE_ADD_TO_CART "<input type=\"text\" name=\"products_id[" $listing->fields['products_id'] . "]\" value=\"0\" size=\"4\" />";
            }
          } else {
// qty box with add to cart button
            
if (PRODUCT_LIST_PRICE_BUY_NOW == '2' && $listing->fields['products_qty_box_status'] != 0) {
              
$lc_buttonzen_draw_form('cart_quantity'zen_href_link(zen_get_info_page($listing->fields['products_id']), zen_get_all_get_params(array('action')) . 'action=add_product&products_id=' $listing->fields['products_id']), 'post''enctype="multipart/form-data"') . '<input type="text" name="cart_quantity" value="' . (zen_get_buy_now_qty($listing->fields['products_id'])) . '" maxlength="6" size="4" /><br />' zen_draw_hidden_field('products_id'$listing->fields['products_id']) . zen_image_submit(BUTTON_IMAGE_IN_CARTBUTTON_IN_CART_ALT) . '</form>';
            } else {
              
$lc_button '<a href="' zen_href_link($_GET['main_page'], zen_get_all_get_params(array('action')) . 'action=buy_now&products_id=' $listing->fields['products_id']) . '">' zen_image_button(BUTTON_IMAGE_BUY_NOWBUTTON_BUY_NOW_ALT'class="listingBuyNowButton"') . '</a>';
            }
          }
        }
        
$the_button $lc_button;
        
$products_link '<a href="' zen_href_link(zen_get_info_page($listing->fields['products_id']), ($_GET['cPath'] > 'cPath=' $_GET['cPath'] . '&' '') . 'products_id=' $listing->fields['products_id']) . '">' MORE_INFO_TEXT '</a>';
        
$lc_text .= '<br />' zen_get_buy_now_button($listing->fields['products_id'], $the_button$products_link) . '<br />' zen_get_products_quantity_min_units_display($listing->fields['products_id']);
        
$lc_text .= '<br />' . (zen_get_show_product_switch($listing->fields['products_id'], 'ALWAYS_FREE_SHIPPING_IMAGE_SWITCH') ? (zen_get_product_is_always_free_shipping($listing->fields['products_id']) ? TEXT_PRODUCT_FREE_SHIPPING_ICON '<br />' '') : '');
 
 
        break;
        case 
'PRODUCT_LIST_QUANTITY':
        
$lc_align 'right';
        
$lc_text $listing->fields['products_quantity'];
        break;
        case 
'PRODUCT_LIST_WEIGHT':
        
$lc_align 'right';
        
$lc_text $listing->fields['products_weight'];
        break;
        case 
'PRODUCT_LIST_IMAGE':
        
$lc_align 'center';
        if (
$listing->fields['products_image'] == '' and PRODUCTS_IMAGE_NO_IMAGE_STATUS == 0) {
          
$lc_text '';
        } else {
          if (isset(
$_GET['manufacturers_id'])) {
            
$lc_text '<a href="' zen_href_link(zen_get_info_page($listing->fields['products_id']), 'products_id=' $listing->fields['products_id']) . '">' zen_image(DIR_WS_IMAGES $listing->fields['products_image'], $listing->fields['products_name'], IMAGE_PRODUCT_LISTING_WIDTHIMAGE_PRODUCT_LISTING_HEIGHT'class="listingProductImage"') . '</a>';
          } else {
            
$lc_text '<a href="' zen_href_link(zen_get_info_page($listing->fields['products_id']), ($_GET['cPath'] > 'cPath=' $_GET['cPath'] . '&' '') . 'products_id=' $listing->fields['products_id']) . '">' zen_image(DIR_WS_IMAGES $listing->fields['products_image'], $listing->fields['products_name'], IMAGE_PRODUCT_LISTING_WIDTHIMAGE_PRODUCT_LISTING_HEIGHT'class="listingProductImage"') . '</a>';
          }
        }
        break;
      }
      
$list_box_contents[$rows][$col] = array('align' => $lc_align,
                                              
'params' => 'class="productListing-data"',
                                              
'text'  => $lc_text);
 
    }
    
// add description and match alternating colors
    //if (PRODUCT_LIST_DESCRIPTION > 0) {
    //  $rows++;
    //  if ($extra_row == 1) {
    //    $list_box_description = "productListing-data-description-even";
    //    $extra_row=0;
    //  } else {
    //    $list_box_description = "productListing-data-description-odd";
    //    $extra_row=1;
    //  }
    //  $list_box_contents[$rows][] = array('params' => 'class="' . $list_box_description . '" colspan="' . $zc_col_count_description . '"',
    //  'text' => zen_trunc_string(zen_clean_html(stripslashes(zen_get_products_description($listing->fields['products_id'], $_SESSION['languages_id']))), PRODUCT_LIST_DESCRIPTION));
    //}
 
 
    
$listing->MoveNext();
  }
 
  
$error_categories false;
} else {
  
$list_box_contents = array();
  
$list_box_contents[0] = array('params' => 'class="productListing-odd"');
  
$list_box_contents[0][] = array('params' => 'class="productListing-data"',
                                              
'text' => TEXT_NO_PRODUCTS);
  
$error_categories true;
}
if ((
$how_many and $show_submit == true and $listing_split->number_of_rows 0) and (PRODUCT_LISTING_MULTIPLE_ADD_TO_CART == or  PRODUCT_LISTING_MULTIPLE_ADD_TO_CART == 3) ) {
  
$show_top_submit_button true;
} else {
  
$show_top_submit_button false;
}
if ((
$how_many and $show_submit == true and $listing_split->number_of_rows 0) and (PRODUCT_LISTING_MULTIPLE_ADD_TO_CART >= 2) ) {
  
$show_bottom_submit_button true;
} else {
  
$show_bottom_submit_button false;
}
 
  if (
$how_many && PRODUCT_LISTING_MULTIPLE_ADD_TO_CART != and $show_submit == true and $listing_split->number_of_rows 0) {
  
// bof: multiple products
    
echo zen_draw_form('multiple_products_cart_quantity'zen_href_link(FILENAME_DEFAULTzen_get_all_get_params(array('action')) . 'action=multiple_products_add_product'), 'post''enctype="multipart/form-data"');
  }
 
?>
As an aside, is there any documentation on the interaction of all the files?? I would be interested in helping work on this if there is not, as I think it would help a lot of users - me included!!

Thanks for any advice any may have.