You are already in PHP in the block where you pasted it. So no need for the start and end php - instead use

$show_display_category = $db->Execute(SQL_SHOW_PRODUCT_INFO_MAIN);
require($template->get_template_dir('tpl_modules_seasonal.php',DIR_WS_TEMPLATE, $current_page_base,'templates'). '/tpl_modules_seasonal.php');
while (!$show_display_category->EOF) {
...