مشكل في تسجيل الدخول

<?php
session_start();
if (isset($_POST['username'])){
$username = htmlentities($_POST['username']);
$password = htmlentities($_POST['password']);
$con = mysql_connect('localhost','root','') or die ("error serveur connect");
$data = mysql_select_db('forum') or die ("error database connect");
$_pass=($password);
$sql = "select * from users where username = '$username'";
$query = mysql_query($sql);
if(!$query){
echo mysql_error();
}
$num = mysql_num_rows($query);
if($num > 0){
while ($resultat = mysql_fetch_assoc($query))
{
	$dbid = $resultat['id'];
	$dbusername = $resultat['username'];	
	$dbpassword = $resultat['password'];

}
if ($username == $dbusername and $password == $dbpassword){
if(!isset($_SESSION['username'])){
$_SESSION['username'] = $username ;
}
header("location: index.php");}
else{ 
echo 'password incorrect';}
}
else {echo 'user not register';}
}
else {
echo 'please enter all information ';}
?>