Не получается сниппет

<?php
$rn_idcar = isset ($idcar);
if ($rn_idcar) {
$rn_tablecar = $modx->getFullTableName("site_content");
$rn_allcars = $modx->db->select("id, pagetitle, description, parent, alias, published, introtext", $rn_tablecar, "parent = $rn_idcar", "id DESC", "0, 10");
if( $modx->db->getRecordCount( $rn_allcars ) >= 1 ) {
		$output .= '<ul>';
		while( $carrow = $modx->db->getRow( $rn_allcars ) ) {
			$output .= '<li>ID: ' . $carrow['id'] . ' | Alias: ' . $carrow['alias'] . '</li>';
		}
		$output .= '</ul>';
	}
	else {
		$output = 'Нету автомобилей';
	}
	return $output;
}
?>

[[Cars?idcar=4]]

[[Cars? &idcar=`4`]]