if (isset($_GET['id'])) {
$id = abs(intval($_GET['id']));
$res = mysql_query("SELECT * FROM `table` WHERE `id` = '$id'");
}как-то так
if (isset($_GET['id'])) {
$id = abs(intval($_GET['id']));
$res = mysql_query("SELECT * FROM `table` WHERE `id` = '$id'");
}