Listbox จำค่าข้อมูลเดิมเสมอ ไม่ทราบว่าต้องแก้ไขอย่างไรครับ

<?
session_register("office","isic");

require("connect_search_90.php");

if (($office == "0") and ($isic == "0"))
{
	header("Location: search_result1.php");
}
elseif (($office == "0") and ($isic != "0"))
{
	header("Location: search_result2.php");
}
elseif (($office != "0") and ($isic == "0"))
{
	header("Location: search_result3.php");
}
elseif (($office != "0") and ($isic != "0"))
{
	header("Location: search_result4.php");
}
?>