ติดปัญหา login ไม่ผ่าน admin check ทั้งที่ไม่Error

<form action="admin_check.php"method="post">
<table width="300"align="center">
<tr>
<td width="100">ชื่อผู้ใช้</td>
<td><input type="text"name="user"size="20"></td>
</tr>
<tr>
<td width="100">รหัสผ่าน</td>
<td><input type="password" name="pass" size="20"></td>
</tr>
<tr>
<td width="100"></td>
<td><input type="submit"value="Login">
<input type="button"value="Cancel"onclick="history.back(0)">
</td>

<?php
$hostname="localhost";
$username="root";
$password="root";
$dbname="test";
$conn=mysql_connect($hostname,$username,$password);
	if(!$conn)
		die('can not connect MySQL');
	mysql_select_db($dbname,$conn)
		or die ('can not select database');
?>

#admin_check
?
	include "connect.php";
	session_start();
	$user = $_POST["user"];
	$pass = $_POST["pass"];
	$sql = "select * from login";
	$ex = mysql_query($sql, $conn);
	while ($rs = mysql_fetch_array($ex)){
		if (($user==$rs["username"]) and ($pass==$rs["password"])) {
			$_SESSION["username"] = $user;
			$_SESSION["password"] = $pass;
			$show = header("location:admin_home.php");
			} else {
			$show = "<center><font face='th sarabun new' size='+2'>Your username or password is invalid<br>
							<a href='admin.html'>กรุณากรอกข้อมูลใหม่อีกครั้ง</font></a>";
			}
			}
		echo $show;
	mysql_close($conn);
?>

#admin_home
<?
	session_start();
	if ((empty($_SESSION["username"])) or (empty($_SESSION["password"]))){
	header("location:admin.html");
	exit;
}
	include"admin_menu.php";
	echo "<br><br><center><H1>ระบบร้านค้าออนไลน์</H1></center>";
?>