PHP เขียน code แล้วขึ้นNotice: ... แต่ยังรันโค้ดได้ครับ

<?php
$tt = $_GET['room'];
$a1 = one;
$a2 = tow;
$a3 = ok;
$a4 = yoyo;
$a5 = E;
$a6 = pop;
if($tt==$a1){
	$titleroom = "1";
}
elseif($tt==0){
	$titleroom = "ไม่พบห้องที่คุณขอ";
	echo"<meta http-equiv='refresh' content='0;URL=index.php' />";
}
elseif($tt==$a2){
	$titleroom = "2";
}
elseif($tt==$a3){
	$titleroom = "3";
}
elseif($tt==$a4){
	$titleroom = "4";
}
elseif($tt==$a5){
	$titleroom = "5";
}
elseif($tt==$a6){
	$titleroom = "6";
}
?>