ปัญหาเรื่องการเช็ค user อะครับ ผมมี user อยู่ 3 ระดับ

<?
session_start();
$username=$_POST[username];
$password=$_POST[password];
/*
if ($username==""and $password=="")
{
	echo "<h3>ERROR : ¡ÃسҡÃÍ¡¢éÍÁÙÅãËé¤Ãº¹Ð¤èÐ</h3>";
	exit();
}*/

include "connect.php";
	$sql="SELECT * FROM user WHERE username='$username' and password='$password'" ;
	//echo $sql;
	
	$result=mysql_db_query($dbname,$sql)or die(mysql_error());
	$num_rows=mysql_num_rows($result);
	//mysql_close();	
	if($num_rows==0){
	 // echo "<h3>ERROR : Username ËÃ×Í Password äÁè¶Ù¡µéͧ </h3>";
	}else{
				
	$_SESSION[sess_userid]=session_id();
	$_SESSION[sess_username]=$username;
	$arr = mysql_fetch_array($result);
				
	$username=$arr["username"];
	$user_type=$arr["user_type"];


	echo"¤Ø³    		$user_name       ";
	echo"    		$user_surname<br>";
	echo"µÓáË¹è§     	$user_type<br>";

	if($user_type == "admin"){
	$_SESSION['username']=$username;
	$_SESSION['stu_name']=$stu_name;
	$_SESSION['stu_surname']=$stu_surname;
	$_SESSION['no_major']=$no_major;
	$_SESSION['tea_name']=$tea_name;
	$_SESSION['tea_surname']=$tea_surname;
	$_SESSION['user_type']=$user_type;
	header("location:admin_index.php");

	}elseif($user_type=="student"){
	$_SESSION['username']=$username;
	$_SESSION['stu_name']=$stu_name;
	$_SESSION['stu_surname']=$stu_surname;
	$_SESSION['no_major']=$no_major;
	$_SESSION['tea_name']=$tea_name;
	$_SESSION['tea_surname']=$tea_surname;
	$_SESSION['user_type']=$user_type;
	 header("location:student_index.php");

	}elseif($user_type=="officer"){
	$_SESSION['username']=$username;
	$_SESSION['stu_name']=$stu_name;
	$_SESSION['stu_surname']=$stu_surname;
	$_SESSION['no_major']=$no_major;
	$_SESSION['tea_name']=$tea_name;
	$_SESSION['tea_surname']=$tea_surname;
	$_SESSION['user_type']=$user_type;
	header("location:officer_index.php");
	}elseif($user_type=="company"){
	$_SESSION['username']=$username;
	$_SESSION['stu_name']=$stu_name;
	$_SESSION['stu_surname']=$stu_surname;
	$_SESSION['no_major']=$no_major;
	$_SESSION['tea_name']=$tea_name;
	$_SESSION['tea_surname']=$tea_surname;
	$_SESSION['user_type']=$user_type;
	header("location:company_index.php");

	}elseif($user_type=="teacher"){
	$_SESSION['username']=$username;
	$_SESSION['stu_name']=$stu_name;
	$_SESSION['stu_surname']=$stu_surname;
	$_SESSION['no_major']=$no_major;
	$_SESSION['tea_name']=$tea_name;
	$_SESSION['tea_surname']=$tea_surname;
	$_SESSION['user_type']=$user_type;
	header("location:teacher_index.php");
	}else{
	echo "ERROR : Username ËÃ×Í Password äÁè¶Ù¡µéͧ";
	}
}
?>