function join_product_table($cid = 0){
       global $db;
       $sql  =" SELECT p.id,p.name,p.quantity,p.buy_price,p.sale_price,p.media_id,p.date,c.name";
       $sql  .=" AS categorie,m.file_name AS image";
       $sql  .=" FROM products p";
       $sql  .=" LEFT JOIN categories c ON c.id = p.categorie_id";
       $sql  .=" LEFT JOIN media m ON m.id = p.media_id";
       if ($cid) $sql .= " WHERE p.categorie_id = " . $cid;
       $sql  .=" ORDER BY p.id ASC";
       return find_by_sql($sql);

    }

<?php
$page_title = 'Stoktaki Ürünler';
  require_once('includes/load.php');
  // Checkin What level user has permission to view this page
   page_require_level(2);
  $cid = (int) (isset($_REQUEST['cid']) ? $_GET['cid'] : 0);
  $products = join_product_table($cid);

<div class="pull-right">
            <div>
                <?php
                    $sql = 'SELECT `id`, `name` FROM `categories` ORDER BY `name` ASC';
                    $categories = find_by_sql($sql);
                    echo '<select onchange="if (this.value) window.location.href=\'?cid=\' + this.value;">';
                    foreach ($categories as $category) {
                        echo '<option value="' . $category['id'] . '">' . $category['id'] . '</option>';
                    }
                    echo '</select>';
                ?>
            </div>
           <a href="add_product.php" class="btn btn-primary">Yeni Ekle</a>
         </div>