Find:

        $products_query = tep_db_query("select p.products_id, pd.products_name, p.products_model, p.products_image, p.products_price, p.products_weight, p.products_tax_class_id from " . TABLE_PRODUCTS . " p, " . TABLE_PRODUCTS_DESCRIPTION . " pd where p.products_id = '" . (int)$products_id . "' and pd.products_id = p.products_id and pd.language_id = '" . (int)$languages_id . "'");
        if ($products = tep_db_fetch_array($products_query)) {
          $prid = $products['products_id'];
          $products_price = $products['products_price'];

          $specials_query = tep_db_query("select specials_new_products_price from " . TABLE_SPECIALS . " where products_id = '" . (int)$prid . "' and status = '1'");
          if (tep_db_num_rows($specials_query)) {
            $specials = tep_db_fetch_array($specials_query);
            $products_price = $specials['specials_new_products_price'];
          }

Change to:

        if ($products = $pf->loadProduct($products_id, $languages_id)) {
          $products_price = $pf->computePrice($this->contents[$products_id]['qty']);

<?php
/*
  $Id: PriceFormatter.php,v 1.6 2003/06/25 08:29:26 petri Exp $

  osCommerce, Open Source E-Commerce Solutions
  http://www.oscommerce.com

  Copyright (c) 2003 osCommerce

  Released under the GNU General Public License
*/

/*
    PriceFormatter.php - module to support quantity pricing

    Created 2003, Beezle Software based on some code mods by WasaLab Oy (Thanks!)
*/

class PriceFormatter {
  var $hiPrice;
  var $lowPrice;
  var $quantity;
  var $hasQuantityPrice;

  function PriceFormatter($prices=NULL) {
    $this->productsID = -1;

    $this->hasQuantityPrice=false;
    $this->hasSpecialPrice=false;

    $this->hiPrice=-1;
    $this->lowPrice=-1;

    for ($i=1; $i<=8; $i++){
      $this->quantity[$i] = -1;
      $this->prices[$i] = -1;
    }
    $this->thePrice = -1;
    $this->specialPrice = -1;
    $this->qtyBlocks = 1;

    if($prices)
      $this->parse($prices);
  }

  function encode() {
    $str = $this->productsID . ":"
           . (($this->hasQuantityPrice == true) ? "1" : "0") . ":"
           . (($this->hasSpecialPrice == true) ? "1" : "0") . ":"
           . $this->quantity[1] . ":"
           . $this->quantity[2] . ":"
           . $this->quantity[3] . ":"
           . $this->quantity[4] . ":"
           . $this->quantity[5] . ":"
           . $this->quantity[6] . ":"
           . $this->quantity[7] . ":"
           . $this->quantity[8] . ":"
           . $this->price[1] . ":"
           . $this->price[2] . ":"
           . $this->price[3] . ":"
           . $this->price[4] . ":"
           . $this->price[5] . ":"
           . $this->price[6] . ":"
           . $this->price[7] . ":"
           . $this->price[8] . ":"
           . $this->thePrice . ":"
           . $this->specialPrice . ":"
           . $this->qtyBlocks . ":"
           . $this->taxClass;
    return $str;
  }

  function decode($str) {
    list($this->productsID,
         $this->hasQuantityPrice,
         $this->hasSpecialPrice,
         $this->quantity[1],
         $this->quantity[2],
         $this->quantity[3],
         $this->quantity[4],
         $this->quantity[5],
         $this->quantity[6],
         $this->quantity[7],
         $this->quantity[8],
         $this->price[1],
         $this->price[2],
         $this->price[3],
         $this->price[4],
         $this->price[5],
         $this->price[6],
         $this->price[7],
         $this->price[8],
         $this->thePrice,
         $this->specialPrice,
         $this->qtyBlocks,
         $this->taxClass) = explode(":", $str);

    $this->hasQuantityPrice = (($this->hasQuantityPrice == 1) ? true : false);
    $this->hasSpecialPrice = (($this->hasSpecialPrice == 1) ? true : false);
  }

  function parse($prices) {
    $this->productsID = $prices['products_id'];
    $this->hasQuantityPrice=false;
    $this->hasSpecialPrice=false;

    $this->quantity[1]=$prices['products_price1_qty'];
    $this->quantity[2]=$prices['products_price2_qty'];
    $this->quantity[3]=$prices['products_price3_qty'];
    $this->quantity[4]=$prices['products_price4_qty'];
    $this->quantity[5]=$prices['products_price5_qty'];
    $this->quantity[6]=$prices['products_price6_qty'];
    $this->quantity[7]=$prices['products_price7_qty'];
    $this->quantity[8]=$prices['products_price8_qty'];

    $this->thePrice=$prices['products_price'];
    $this->specialPrice=$prices['specials_new_products_price'];
    $this->hasSpecialPrice=tep_not_null($this->specialPrice);

    $this->price[1]=$prices['products_price1'];
    $this->price[2]=$prices['products_price2'];
    $this->price[3]=$prices['products_price3'];
    $this->price[4]=$prices['products_price4'];
    $this->price[5]=$prices['products_price5'];
    $this->price[6]=$prices['products_price6'];
    $this->price[7]=$prices['products_price7'];
    $this->price[8]=$prices['products_price8'];


     /*
       Change support special prices
       If any price level has a price greater than the special
       price lower it to the special price
    */
    if ($this->hasSpecialPrice == true) {
        for($i=1; $i<=8; $i++) {
            if ($this->price[$i] > $this->specialPrice)
                $this->price[$i] = $this->specialPrice;
        }
    }
    //end changes to support special prices

    $this->qtyBlocks=$prices['products_qty_blocks'];

    $this->taxClass=$prices['products_tax_class_id'];

    if ($this->quantity[1] > 0) {
      $this->hasQuantityPrice = true;
      $this->hiPrice = $this->thePrice;
      $this->lowPrice = $this->thePrice;

      for($i=1; $i<=8; $i++) {
    if($this->quantity[$i] > 0) {
      if ($this->price[$i] > $this->hiPrice) {
        $this->hiPrice = $this->price[$i];
      }
      if ($this->price[$i] < $this->lowPrice) {
        $this->lowPrice = $this->price[$i];
      }
    }
      }
    }
  }

  function loadProduct($product_id, $language_id=1)
  {
    $sql="select pd.products_name, p.products_model, p.products_image, p.products_id," .
        " p.manufacturers_id, p.products_price, p.products_weight," .
        " p.products_price1,p.products_price2,p.products_price3,p.products_price4, p.products_price5,p.products_price6,p.products_price7,p.products_price8," .
        " p.products_price1_qty,p.products_price2_qty,p.products_price3_qty,p.products_price4_qty, p.products_price5_qty,p.products_price6_qty,p.products_price7_qty,p.products_price8_qty," .
        " p.products_qty_blocks," .
        " p.products_tax_class_id," .
        " IF(s.status, s.specials_new_products_price, NULL) as specials_new_products_price," .
        " IF(s.status, s.specials_new_products_price, p.products_price) as final_price" .
        " from " . TABLE_PRODUCTS_DESCRIPTION . " pd," .
        "      " . TABLE_PRODUCTS . " p left join " . TABLE_MANUFACTURERS . " m on p.manufacturers_id = m.manufacturers_id," .
        "      " . TABLE_PRODUCTS_TO_CATEGORIES . " p2c left join " . TABLE_SPECIALS . " s on p.products_id = s.products_id" .
        " where p.products_status = '1'" .
        "   and p.products_id = '" . (int)$product_id . "'" .
        "   and pd.products_id = '" . (int)$product_id . "'" .
        "   and pd.language_id = '". (int)$language_id ."'";

    $product_info_query = tep_db_query($sql);
    $product_info = tep_db_fetch_array($product_info_query);
    $this->parse($product_info);

    return $product_info;
  }

  function computePrice($qty)
  {
    $qty = $this->adjustQty($qty);

    // Compute base price, taking into account the possibility of a special
    $price = ($this->hasSpecialPrice === TRUE) ? $this->specialPrice : $this->thePrice;

    for ($i=1; $i<=8; $i++)
        if (($this->quantity[$i] > 0) && ($qty >= $this->quantity[$i]))
            $price = $this->price[$i];

    return $price;
  }

  function adjustQty($qty) {
    // Force QTY_BLOCKS granularity
    $qb = $this->getQtyBlocks();
    if ($qty < 1)
        $qty = 1;

    if ($qb >= 1)
    {
        if ($qty < $qb)
            $qty = $qb;

        if (($qty % $qb) != 0)
            $qty += ($qb - ($qty % $qb));
    }
    return $qty;
  }

  function getQtyBlocks() {
    return $this->qtyBlocks;
  }

  function getPrice() {
    return $this->thePrice;
  }

  function getLowPrice() {
    return $this->lowPrice;
  }

  function getHiPrice() {
    return $this->hiPrice;
  }

  function hasSpecialPrice() {
    return $this->hasSpecialPrice;
  }

  function hasQuantityPrice() {
    return $this->hasQuantityPrice;
  }

  function getPriceString($style='productPriceInBox') {
    global $currencies;

    if ($this->hasSpecialPrice == true) {
        $lc_text = '<table align="top" border="1" bordercolor="#FFFFFF" cellspacing="0" width="350" cellpadding="0">';
        $lc_text .= '<tr><td align="justify" class=' . $style. ' colspan="2"><b></b></td></tr>';
    }
    else
    {
        $lc_text = '<table align="top" border="1" bordercolor="#FFFFFF" cellspacing="0" width="350" cellpadding="0">';
        $lc_text .= '<tr><td align="justify" class=' . $style. ' colspan="2"><b></b></td></tr>';
    }
      // If you want to change the format of the price/quantity table
      // displayed on the product information page, here is where you do it.

    if($this->hasQuantityPrice == true) {
        for($i=1; $i<=8; $i++) {
            if($this->quantity[$i] > 0) {
                $lc_text .= '<tr><td class='.$style.'>&nbsp;&nbsp;'
                . $this->quantity[$i]
                .'+&nbsp;</td><td class='.$style.'>&nbsp;&nbsp;'
                . $currencies->display_price($this->price[$i],
                tep_get_tax_rate($this->taxClass))
                .'</td></tr>';
            }
        }

        $lc_text .= '</table>';

      }
      else {
        if ($this->hasSpecialPrice == true) {
            $lc_text = '&nbsp;<s>'
              . $currencies->display_price($this->thePrice, tep_get_tax_rate($this->taxClass))
              . '</s>&nbsp;&nbsp;<span class="productSpecialPrice">'
              . $currencies->display_price($this->specialPrice, tep_get_tax_rate($this->taxClass))
              . '</span>&nbsp;';
        }
        else {
            $lc_text = '&nbsp;'
              . $currencies->display_price($this->thePrice,
                       tep_get_tax_rate($this->taxClass))
              . '&nbsp;';
        }
          }

    return $lc_text;
  }

  function getPriceStringShort() {
    global $currencies;

    if ($this->hasSpecialPrice == true) {
      $lc_text = '&nbsp;<s>'
    . $currencies->display_price($this->thePrice,
                     tep_get_tax_rate($this->taxClass))
    . '</s>&nbsp;&nbsp;<span class="productSpecialPrice">'
    . $currencies->display_price($this->specialPrice,
                     tep_get_tax_rate($this->taxClass))
    . '</span>&nbsp;';
    }
    else {
      if($this->hasQuantityPrice == true) {
    $lc_text = '&nbsp;'
      . $currencies->display_price($this->lowPrice,
                       tep_get_tax_rate($this->taxClass))
      . ' - '
      . $currencies->display_price($this->hiPrice,
                       tep_get_tax_rate($this->taxClass))
      . '&nbsp;';
      }
      else {
    $lc_text = '&nbsp;'
      . $currencies->display_price($this->thePrice,
                       tep_get_tax_rate($this->taxClass))
      . '&nbsp;';
      }
    }
    return $lc_text;
  }
}

?>

                   <?php if (($product_info['products_price1'])>0) { ?>


                        
                    <tr><td class="main" align="center"><b><?php echo TEXT_PRIX_DEGRESSIFS; ?></b></td></tr>
                   <tr><td class="main" align="center"><?php echo $products_price_qte; ?></td></tr>


                         </td></tr>
                         <tr><td><?php echo tep_draw_separator('pixel_trans.gif', '100%', '10'); ?></td></tr>
                         <tr><td colspan=3 align=center><img src=images/m33.gif width=469 height=1></td></tr>
                         <tr><td><?php echo tep_draw_separator('pixel_trans.gif', '100%', '10'); ?></td></tr>

                        
                           <?php

                        }  else {   }?>