Product Database Query - Narrow the results

In the file / view product / listing_div, I wish to narrow the results of the category database query so that custom product fields are reflected in the query.

I want to modify the code that displays the products in a particular category (below). How can I narrow the products displayed based on a particular value of a custom product field (for instance, WHERE xyz = "abc")?:
foreach($this->rows as $row){
if (
?>
<div class="hikashop_product" style="width:<?php echo $width;?>%;">
<div class="hikashop_container">
<div class="hikashop_subcontainer">
<?php
$this->row =& $row;
$this->setLayout('listing_'.$this->params->get('div_item_layout_type'));
echo $this->loadTemplate();
?>
</div>
</div>
</div>
<?php if($current_column>=$columns){ ?>
<div style="clear:both"></div>
<?php
$current_column=0;
}
$current_column++;
}

Thanks for your quick response and help. I give it a whirl.

I've added the plugin, but the filter is ignored on the front end and the back end throws an error:

Warning: Invalid argument supplied for foreach() in /home/.../administrator/components/com_hikashop/views/dashboard/view.html.php on line 423

I don't think I'm constructing the filter value properly. Can you advise?

Here is plugin:
<?php
defined('_JEXEC') or die('Restricted access');
?>
<?php

class plgHikashopCategoryFilters extends JPlugin{

function plgHikashopCategoryFilters(&$subject, $config){
parent::__construct($subject, $config);
}

function onBeforeCategoryListingLoad(&$filters){

$ageval = $_POST;
$locval = $_POST;
$typeval = $_POST;
$filters[0] = "(age = '$ageval') AND (type = '$typeval') AND (location = '$locval')";
//echo $filters[0];
return true;
}
}

Thanks for the help.

I changed it to this but it still does not seem to have any effect on front end and still get same warning on back end. These are custom fields defined for the product. Any ideas? :

<?php
defined('_JEXEC') or die('Restricted access');
?>
<?php

class plgHikashopCategoryFilters extends JPlugin{

function plgHikashopCategoryFilters(&$subject, $config){
parent::__construct($subject, $config);
}

function onBeforeCategoryListingLoad(&$filters){

if(isset($_POST)) { $age = stripslashes($_POST); } else { $age = "All";};
if(isset($_POST)) { $location = stripslashes($_POST); } else { $location = "All";};
if(isset($_POST)) { $type = stripslashes($_POST); } else { $type = "All";};

$db =& JFactory::getDBO();
$filters[] = "age = '".$db->Quote($age)."'";
$filters[] = "type = '".$db->Quote($type)."'";
$filters[] = "location = '".$db->Quote($location)."'";
echo $filters[0];
echo $filters[1];
echo $filters[2];
echo $filters[3];
echo $filters[4];
return true;
}
}

HI,

Here is message in back end when clicking on Components Hikashop (message goes away when I disable the plugin):

a.category_parent_id = 4age = ''All''type = ''All''location = ''All''trans =
Warning: Invalid argument supplied for foreach() in /administrator/components/com_hikashop/views/dashboard/view.html.php on line 423
Warning: Invalid argument supplied for foreach() in /administrator/components/com_hikashop/views/dashboard/view.html.php on line 423
Warning: Invalid argument supplied for foreach() in /administrator/components/com_hikashop/views/dashboard/view.html.php on line 423
Warning: Invalid argument supplied for foreach() in /administrator/components/com_hikashop/views/dashboard/view.html.php on line 423
Warning: Invalid argument supplied for foreach() in /administrator/components/com_hikashop/views/dashboard/view.html.php on line 423
Warning: Invalid argument supplied for foreach() in /administrator/components/com_hikashop/views/dashboard/view.html.php on line 423
a.category_parent_id = 4age = ''All''type = ''All''location = ''All''trans =


Here is XML file, maybe I'm missing something:

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE install SYSTEM " dev.joomla.org/xml/1.5/plugin-install.dtd ">
<install type="plugin" version="1.5" method="upgrade" group="hikashop">
<name>Hikashop Category Filters Plugin</name>
<creationDate>July 2011</creationDate>
<version>1.0.0</version>
<author>Sullivan</author>
<authorEmail>This email address is being protected from spambots. You need JavaScript enabled to view it.</authorEmail>
<authorUrl> www.gophercreative.com </authorUrl>
<copyright>Copyright (C) 2011</copyright>
<license> www.gnu.org/licenses/gpl-2.0.html GNU/GPL</license>
<description>This plugin adds extra filters to category selection</description>
<files>
<filename plugin="categoryfilters">categoryfilters.php</filename>
</files>
<params addpath="/components/com_hikashop/params">
</params>
</install>

Same thing except now values displayed with echo command have single quotes around them instead of double quotes.

Thanks very much for finding my mistake. Works great now!